Themes, typography and image proxy

I need to move forward. It’s very inconvenient to do all this customization without CSS variables (which are supported by all major browsers for at least 2-3 years). I don’t want to write hacky incomprehensible code just to support old devices that used by less than 1% of users (I’ve checked usage before making this decision). Even major sites (and even Microsoft itself) no longer support Internet Explorer.