Simplify theme color customisation

[KittyChiu]

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (refactoring and improving code)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Description

Allow customisation to the theme colors with simple _config.yml settings, rather than editing the css.

Additional context

#1354 (pls reopen issue)

How to test

1. Update _config.yml and update values under theme_colors. For example

theme_colors:
  light:
    forecolor: "#444400" # e.g. "#34343c"
    backcolor: "#ffffff" # e.g. "#ffffff"
  dark:
    forecolor: "#ffffff" # e.g. "#afb0b1"
    backcolor: "#330033" # e.g. "#1b1b1e"

[cotes2020]

Hi there! Just wanted to check—are you looking to just update the colors for those two <meta> tags? Or is the plan to support an arbitrary number of custom colors via _config.yml?

[KittyChiu]

The intent is to support custom colour via _config.yml

[cotes2020]

Automatically calculating the remaining colors results in a poor UI experience. I recommend injecting CSS to modify the existing colors instead of relying on that method.