Use design tokens in Bootstrap reset

Problem to solve

• GitLab UI is built on top of BootstrapVue, which is itself built on top of Bootstrap CSS.
• Bootstrap sets a host of global styles using their own variables system and global resets.
• These definitions make it difficult for us to apply design tokens to our Pajamas components.

Proposal

• Apply design tokens to Bootstrap CSS reset (or reimplement with our own reset)
• Apply design tokens to Bootstrap CSS variables Split into a separate issue: #3168 (moved)

Next steps

• Remove or replace -webkit-tap-highlight-color from _reboot.scss