CSS variables
Every developer who has developed any webpage has the same problem: a change of primary color implies changes in many CSS files and careful find/replace operations for all occurrences of specific value.
Some of us are already using CSS pre-compiler libraries (like Less), which allows us using variables on CSS level in our code.
Less can be embedded strictly using client-side pre-compilation, without a need to compute it on application server (like PHP or ASP scripts), which would definitely make it easier to start using Less stylesheets in our project.