I use Gulp as a build tool to automate a lot of the mundane tasks involved with creating WordPress themes and there are many Gulp packages to take care of everything from generating translation files, to compiling SCSS into CSS.
- cssmqpacker: combines matching media queries into single sections.
- autoprefixer: adds browser prefixes for your CSS rules, based on the range of supported browsers you feed in.
- pxtorem: converts static pixel values into flexible rem values.
- cssnano: compresses and minifies your generated CSS into the smallest production-ready file possible.