Custom architecture from scratch, sometimes based on Bootstrap 4 which is still in Alpha and I am contributing to. Bootstrap is the most popular product and reached 100k stars this year. You may also learn a lot from mdo and team.
If you want to save a bit of your time then I would recommend Bootstrap 4 which is in active development, uses SASS now and has huge variables.scss and customization options starting from old layout / new flex layout till brand colors, enabling/disabling shadows, animations, etc.