[Lookup Anything] Background theme options

[Mushymato]

Adds some background theme options, and refactor code for potential mod added themes down the road (i.e. not in this PR)

[Pathoschild]

Thanks for the pull request!

Splitting the logic between LetterBackground + MenuBoxBackground + PlainBackground will make things harder to extend later though. Could you combine them into one implementation which reads a unified data model?

For example, something like...

/// <summary>Get the default themes.</summary>
private static Dictionary<string, ThemeData> GetDefaultThemes()
{
    return new()
    {
        // formerly LetterBackground
        ["TornLetter"] = new ThemeData
        {
            Background = new()
            {
                Texture = "LooseSprites/letterBG",
                SourceRect = new Rectangle(0, 0, 320, 180)
            }
        },

        // formerly PlainBackground
        ["PlainWheat"] = new ThemeData
        {
            Background = new()
            {
                Color = "Wheat"
            },
            Border = new()
            {
                Color = "BurlyWood",
                Width = 4
            }
        },

        // formerly MenuBoxBackground
        ["MenuBox_Inset"] = new ThemeData
        {
            Background = new()
            {
                Texture = "Maps/MenuTiles",
                SourceRect = new Rectangle(0, 316, 60, 60),
                ScaleMode = BackgroundScaleMode.TextureBox
            }
        }
    };
}

Then the unified implementation would do something like...

public void DrawBackground(SpriteBatch b, int x, int y, int width, int height)
{
    if (this.Theme.Background is { } background)
    {
        if (background.Color != null)
            Utility.DrawSquare(b, new(x, y, width, height), 0, backgroundColor: background.Color);

        if (background.Texture != null)
        {
            Rectangle sourceRect = background.SourceRect;

            switch (background.ScaleMode)
            {
                case BackgroundScaleMode.TextureBox:
                    // copy logic from MenuBoxBackground
                    break;
                    
                default:
                    // copy logic from LetterBackground
                    break;
            }
        }
    }

   if (this.Theme.Border is { } border)
       Utility.DrawSquare(b, new(x, y, width, height), border.Width, border.Color);
}

Once we have that, it'll be much easier to allow custom mod themes (which I could add after merging the PR).