I would look at adding SCSS to your workflow. It makes writing maintainable stylesheets much easier. The two biggest wins from using SCSS, in my opinion, are the variables and the ability to nest selectors.

The node-sass package (on NPM) comes with a command line tool, so you don't even need to add a separate build tool.

I have an NPM script that executes:

    node-sass -w styles/app.scss -o dist/

The -w flag tells the tool to stay in "watch" mode, such that it will auto recompile your SCSS code into CSS whenever a change is made.