[CSS] Cleaned up Bootstrap overrides

Now that we're using Bootstrap-SASS, we no longer need to replicate a lot of the variables and mixins to make the overrides work. Also, we can now override the values of the Bootstrap variables in WET's _variables.scss instead of replicating the SCSS code that we want to override.

This also reduces the final size of the wet-boew.css by about 6 KB.