User blog comment:JoePlay/Creating Your Wiki’s Design/@comment-5645428-20171217003808

Any person aspiring to customise their wiki using CSS should take serious note of one of the newer features at their disposal, CSS custom variables. Gone are the days needing to scour entire stylesheets replacing countless numbers of values to perform an update to the colour scheme. Now, all colour values can be specified in a centralised location that rolls down across entire stylesheets with efficiency and can be changed immediately at will. Especially helpful for those interested in implementing temporary seasonal themes.

https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables

Browser support is widespread (of course IE misses though) and will only faze those who have a radical level of concern for overall compatibility.

https://caniuse.com/#feat=css-variables

Good luck!