Remove redundant `bootstrap` SCSS variables

Created by: valerybugakov

Problem statement

We removed all bootstrap imports recently, but we still have a lot of SCSS variables that are meant to redefine bootstrap internal variables. Most of them are redundant now, and we can safely remove them to simplify our global CSS setup.

Success criteria

Redundant SCSS variables are removed or replaced with CSS variables if they are still in use.

Time estimate

• Pull requests with ~450 lines changed should take 4 hours at maximum. Ping the reviewer in the spec pull request if time-consuming changes are required.
• Split the work into multiple pull requests if the total diff is bigger than 450 lines of code.