Use CSS vars to homogenize UI style

[le-jeu]

Thanks to CSS, we can use variables to:

• avoid repetition of a color with the same semantic in IITC style
• expose colors for plugins to use instead of copy/paste
• make style (color) customization simpler (from plugins or in order to make a darktheme)

a purple lover could use

:root {
  --ui-background-color: purple;
  --highlight-text-color: yellow;
}

while a dark mode could looks like

@media (prefers-color-scheme: dark) {
 :root {
  --ui-background-color: #000e;
  --highlight-text-color: #ccc;
  --ui-border-color: #000;
  --ui-background-color-no-alpha: #000;
  --ui-title-background-color: #0008;
  --highlight-border-color: #fff;
  --arrow-toggle-color: #fff; 
 }
}

[johnd0e]

https://caniuse.com/css-variables

Android 4.4 (KitKat) WebView does not support it.
As well as PhanthomJS (not sure that we still care though).

In general: I'd prefer to refactor whole css from scratch first.