From 78165115c5b2b5561dcb67184cc8c0b5ed0da608 Mon Sep 17 00:00:00 2001
From: Richard Iannone <riannone@me.com>
Date: Wed, 1 Oct 2025 15:15:59 -0400
Subject: [PATCH 1/3] Add the opt_css() method

---
 great_tables/_options.py | 97 +++++++++++++++++++++++++++++++++++++++-
 great_tables/_scss.py    | 10 +++--
 great_tables/gt.py       |  2 +
 3 files changed, 104 insertions(+), 5 deletions(-)

diff --git a/great_tables/_options.py b/great_tables/_options.py
index f4ab0fc8a..cda1ed20d 100644
--- a/great_tables/_options.py
+++ b/great_tables/_options.py
@@ -22,7 +22,7 @@ def tab_options(
     table_margin_left: str | None = None,
     table_margin_right: str | None = None,
     table_background_color: str | None = None,
-    table_additional_css: list[str] | None = None,
+    table_additional_css: str | list[str] | None = None,
     table_font_names: str | list[str] | None = None,
     table_font_size: str | None = None,
     table_font_weight: str | int | float | None = None,
@@ -553,6 +553,11 @@ def tab_options(
         if isinstance(modified_args["table_font_names"], str):
             modified_args["table_font_names"] = [modified_args["table_font_names"]]
 
+    # - `table_additional_css` should be a list but if given as a string, ensure it is list
+    if "table_additional_css" in modified_args:
+        if isinstance(modified_args["table_additional_css"], str):
+            modified_args["table_additional_css"] = [modified_args["table_additional_css"]]
+
     new_options_info = {
         k: replace(getattr(self._options, k), value=v) for k, v in modified_args.items()
     }
@@ -1427,6 +1432,96 @@ def dict_omit_keys(dict: dict[str, str], omit_keys: set[str]) -> dict[str, str]:
     return res
 
 
+def opt_css(
+    self: GTSelf,
+    css: str,
+    add: bool = True,
+    allow_duplicates: bool = False,
+) -> GTSelf:
+    """
+    Option to add custom CSS for the table.
+
+    `opt_css()` makes it possible to add extra CSS rules to a table. This CSS will be added after
+    the compiled CSS that Great Tables generates automatically when the object is transformed to an
+    HTML output table.
+
+    Parameters
+    ----------
+    css
+        The CSS to include as part of the rendered table's `<style>` element.
+    add
+        If `True`, the default, the CSS is added to any already-defined CSS (typically from
+        previous calls of `opt_css()` or `tab_options(table_additional_css=...)`). If this is set to
+        `False`, the CSS provided here will replace any previously-stored CSS.
+    allow_duplicates
+        When this is `False` (the default), the CSS provided here won't be added (provided that
+        `add=True`) if it is seen in the already-defined CSS.
+
+    Returns
+    -------
+    GT
+        The GT object is returned. This is the same object that the method is called on so that
+        we can facilitate method chaining.
+
+    Examples
+    --------
+    Let's use the `exibble` dataset to create a simple, two-column table (keeping only the `num` and
+    `currency` columns). Through use of the `opt_css()` method, we can insert CSS rulesets as a
+    string. We need to ensure that the table ID is set explicitly (we've done so here with the ID
+    value of `"one"`, setting it up with `GT(id=)`).
+
+    ```{python}
+    from great_tables import GT, exibble
+    import polars as pl
+
+    exibble_mini = pl.from_pandas(exibble).select(["num", "currency"])
+
+    (
+        GT(exibble_mini, id="one")
+        .fmt_currency(columns="currency", currency="HKD")
+        .fmt_scientific(columns="num")
+        .opt_css(
+            css='''
+            #one .gt_table {
+              background-color: skyblue;
+            }
+            #one .gt_row {
+              padding: 20px 30px;
+            }
+            #one .gt_col_heading {
+              text-align: center !important;
+            }
+            '''
+        )
+    )
+    ```
+    """
+
+    # Get the current additional CSS from `_options.table_additional_css`
+    existing_additional_css: list[str] = self._options.table_additional_css.value or []
+
+    # Convert CSS to a consistent format by stripping any leading or trailing whitespace
+    css = css.strip()
+
+    if not add:
+        # If `add=False`, or if `css` is empty, replace the existing CSS
+        additional_css = [css] if css else []
+    elif not css:
+        # If `css` is empty and we are adding CSS, keep the existing CSS
+        additional_css = existing_additional_css
+    elif not allow_duplicates and css in existing_additional_css:
+        # if CSS already exists and we don't allow duplicates, return self
+        return self
+    else:
+        # Add the new CSS to the existing CSS
+        additional_css = existing_additional_css + [css]
+
+    # Use tab_options() to set the additional CSS
+    res = tab_options(self, table_additional_css=additional_css)
+
+    return res
+
+
 @dataclass
 class StyleMapper:
     table_hlines_color: str
diff --git a/great_tables/_scss.py b/great_tables/_scss.py
index 6d1542452..693458a5a 100644
--- a/great_tables/_scss.py
+++ b/great_tables/_scss.py
@@ -147,7 +147,7 @@ def compile_scss(
         data._google_font_imports.to_css() + "\n" if data._google_font_imports.to_css() else ""
     )
 
-    # Prepend any additional CSS ----
+    # Process any additional CSS that will be appended at the end ----
     additional_css = data._options.table_additional_css.value
 
     # Determine if there are any additional CSS statements
@@ -159,11 +159,11 @@ def compile_scss(
     # separating with `\n`; use an empty string if list is empty or value is None
     if has_additional_css:
         additional_css_unique = OrderedSet(additional_css).as_list()
-        table_additional_css = "\n".join(additional_css_unique) + "\n"
+        table_additional_css = "\n".join(additional_css_unique)
     else:
         table_additional_css = ""
 
-    gt_table_class_str = f"""{table_additional_css}{gt_table_open_str} {{
+    gt_table_class_str = f"""{gt_table_open_str} {{
           {font_family_attr}
           -webkit-font-smoothing: antialiased;
           -moz-osx-font-smoothing: grayscale;
@@ -187,6 +187,8 @@ def compile_scss(
     if all_important:
         compiled_css = re.sub(r";", " !important;", compiled_css, count=0, flags=re.MULTILINE)
 
-    finalized_css = f"{google_font_css}{gt_table_class_str}\n\n{compiled_css}"
+    # Assemble blocks of CSS ----
+    additional_css_block = f"\n{table_additional_css}\n" if has_additional_css else ""
+    finalized_css = f"{google_font_css}{gt_table_class_str}\n\n{compiled_css}{additional_css_block}"
 
     return finalized_css
diff --git a/great_tables/gt.py b/great_tables/gt.py
index e38875950..e8b456a54 100644
--- a/great_tables/gt.py
+++ b/great_tables/gt.py
@@ -36,6 +36,7 @@
 from ._options import (
     opt_align_table_header,
     opt_all_caps,
+    opt_css,
     opt_footnote_marks,
     opt_horizontal_padding,
     opt_row_striping,
@@ -248,6 +249,7 @@ def __init__(
     opt_stylize = opt_stylize
     opt_align_table_header = opt_align_table_header
     opt_all_caps = opt_all_caps
+    opt_css = opt_css
     opt_footnote_marks = opt_footnote_marks
     opt_row_striping = opt_row_striping
     opt_vertical_padding = opt_vertical_padding

From 4044f6081a2365348f557920032d5dff1f464522 Mon Sep 17 00:00:00 2001
From: Richard Iannone <riannone@me.com>
Date: Wed, 1 Oct 2025 15:16:19 -0400
Subject: [PATCH 2/3] Add opt_css() to API reference

---
 docs/_quarto.yml | 1 +
 1 file changed, 1 insertion(+)

diff --git a/docs/_quarto.yml b/docs/_quarto.yml
index f21c42f58..e15beb71a 100644
--- a/docs/_quarto.yml
+++ b/docs/_quarto.yml
@@ -219,6 +219,7 @@ quartodoc:
         - GT.opt_table_outline
         - GT.opt_table_font
         - GT.opt_stylize
+        - GT.opt_css
     - title: Export
       desc: >
         There may come a day when you need to export a table to some specific format. A great method

From b6c78fbe7b540045853809193ccd16312fe79af2 Mon Sep 17 00:00:00 2001
From: Richard Iannone <riannone@me.com>
Date: Wed, 1 Oct 2025 15:16:48 -0400
Subject: [PATCH 3/3] Add several tests of opt_css()

---
 tests/test_options.py | 109 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)

diff --git a/tests/test_options.py b/tests/test_options.py
index 9d274482f..d03805641 100644
--- a/tests/test_options.py
+++ b/tests/test_options.py
@@ -550,3 +550,112 @@ def test_opt_horizontal_padding_raises(gt_tbl: GT, scale: float):
         gt_tbl.opt_horizontal_padding(scale=scale)
 
     assert "`scale` must be a value between `0` and `3`." in exc_info.value.args[0]
+
+
+def test_opt_css_basic():
+    # Test adding CSS
+    res = GT(exibble).opt_css(css=".gt_table { background-color: red; }")
+    assert res._options.table_additional_css.value == [".gt_table { background-color: red; }"]
+
+    # Test chaining CSS additions
+    res_2 = res.opt_css(css=".gt_row { color: blue; }")
+    assert len(res_2._options.table_additional_css.value) == 2
+    assert ".gt_table { background-color: red; }" in res_2._options.table_additional_css.value
+    assert ".gt_row { color: blue; }" in res_2._options.table_additional_css.value
+
+
+def test_opt_css_duplicate_handling():
+    gt_tbl = GT(exibble)
+    css_rule = ".gt_table { color: green; }"
+
+    # Add same CSS twice (should only appear once)
+    res = gt_tbl.opt_css(css=css_rule).opt_css(css=css_rule)
+
+    assert res._options.table_additional_css.value == [css_rule]
+
+    # Test for allowing duplicates
+    res_2 = gt_tbl.opt_css(css=css_rule).opt_css(css=css_rule, allow_duplicates=True)
+
+    assert res_2._options.table_additional_css.value == [css_rule, css_rule]
+
+
+def test_opt_css_replace_mode():
+    # Add initial CSS
+    res = (
+        GT(exibble).opt_css(css=".gt_table { color: red; }").opt_css(css=".gt_row { color: blue; }")
+    )
+
+    assert len(res._options.table_additional_css.value) == 2
+
+    # Replace all CSS with `add=False`
+    res_2 = res.opt_css(css=".gt_table { color: green; }", add=False)
+    assert res_2._options.table_additional_css.value == [".gt_table { color: green; }"]
+
+
+def test_opt_css_whitespace_handling():
+    gt_tbl = GT(exibble)
+
+    # Test using empty-string CSS
+    res = gt_tbl.opt_css(css="")
+    assert res._options.table_additional_css.value == []
+
+    # Test whitespace-only CSS
+    res_2 = gt_tbl.opt_css(css="   \n  \t  ")
+    assert res_2._options.table_additional_css.value == []
+
+    # Test CSS with leading and trailing whitespace (it should get stripped)
+    res_3 = gt_tbl.opt_css(css="  .gt_table { color: red; }  ")
+    assert res_3._options.table_additional_css.value == [".gt_table { color: red; }"]
+
+
+def test_opt_css_multiline():
+    multiline_css = """
+#test_table .gt_table {
+  background-color: skyblue;
+}
+#test_table .gt_row {
+  padding: 20px;
+}"""
+
+    result = GT(exibble, id="test_table").opt_css(css=multiline_css)
+
+    expected_css = multiline_css.strip()
+
+    assert result._options.table_additional_css.value == [expected_css]
+
+
+def test_opt_css_html_output():
+    css_rule = "#test_table .gt_table { background-color: lightblue; }"
+    res = GT(exibble, id="test_table").opt_css(css=css_rule)
+
+    html = res.as_raw_html()
+
+    # Check that the CSS appears in the HTML string
+    assert css_rule in html
+
+    # For the CSS ordering, the added CSS should come after the default CSS
+    default_css_pos = html.find("#test_table .gt_table { display: table;")
+    custom_css_pos = html.find(css_rule)
+
+    assert default_css_pos > 0
+    assert custom_css_pos > 0
+    assert custom_css_pos > default_css_pos
+
+
+def test_opt_css_with_tab_options():
+    res = (
+        GT(exibble)
+        .tab_options(table_additional_css=".initial { color: red; }")
+        .opt_css(css=".added { color: blue; }")
+    )
+
+    css_list = res._options.table_additional_css.value
+
+    assert len(css_list) == 2
+    assert ".initial { color: red; }" in css_list
+    assert ".added { color: blue; }" in css_list
+
+    # Test that `opt_css()` rule comes after the `tab_options()` rule in the rendered HTML
+    html = res.as_raw_html()
+
+    assert html.index(".added { color: blue; }") > html.index(".initial { color: red; }")