I have found Maintainable CSS site exceptionally useful for designing maintainable CSS systems.
Modular and encapsulated: Styles don’t bleed or cascade without your permission.
Any design requirements: Completely flexible to your needs.
No tooling required: But you can use tooling if you want to.
Easy to learn: Read the guides and see.
Any size project: Whatever size project you have, MaintainableCSS will help.
Upgrade in your own time: You can start applying the approach today, bit by bit. You don’t need to upgrade the whole