Add additional-theme option

[expikr]

Closes #2048

This allows users to specify additional themes without replacing the presets.

Example usage

book.toml

[output.html]
additional-css = ["orange.css"]
additional-theme = ["Orange"]

orange.css

Click to expand

.orange {
    --bg: #f6f6ef;
    --fg: Black;

    --sidebar-bg: #ff6600;
    --sidebar-fg: Black;
    --sidebar-non-existant: color-mix(in srgb, var(--sidebar-bg) 75%, Black);
    --sidebar-active: White;
    --sidebar-spacer: color-mix(in srgb, var(--sidebar-bg) 95%, Black);

    --scrollbar: #8F8F8F;

    --icons: #747474;
    --icons-hover: #000000;

    --links: #828282;

    --inline-code-color: Black

    --theme-popup-bg: #fafafa;
    --theme-popup-border: #cccccc;
    --theme-hover: #e6e6e6;

    --quote-bg: #e9e9ed;
    --quote-border: #8f8f9d;

    --table-border-color: hsl(0, 0%, 95%);
    --table-header-bg: hsl(0, 0%, 80%);
    --table-alternate-bg: hsl(0, 0%, 97%);

    --searchbar-border-color: #aaa;
    --searchbar-bg: #fafafa;
    --searchbar-fg: #000;
    --searchbar-shadow-color: #aaa;
    --searchresults-header-fg: #666;
    --searchresults-border-color: #888;
    --searchresults-li-bg: #e4f2fe;
    --search-mark-bg: #a2cff5;
}

Known Issues

• The theme class needs to specify ALL the variables, otherwise they default to no-style.
  Can we figure out an easy way to derive the variable values from the presets?
• The syntax highlighting defaults to that of an assumed light mode (see here).
  What would be the most appropriate way for the theme class to declare itself as a dark-based or light-based theme and have the correct syntax highlight selected?