https://playbook.cio.gov/designstandards/visual-style/
Node: Both of these include a style-enforcement tool that can be scripted via NPM scripts.
For CSS: http://semantic-ui.com/usage/theming.html
Note: Semantic-UI is a lot like Bootstrap in that it provides a lot of useful visual classes/components. In addition, it can be extended to use custom themes.
https://github.com/lipis/awesome-style-guides/blob/master/RE...
https://playbook.cio.gov/designstandards/visual-style/