Convert builtin block Sass variables into CSS variables

[farhan]

In this story we have to convert builtin block Sass variables into CSS variables.

For each builtin block modify its Sass to use the CSS variable instead of the Sass variable.

Tip: Start with one block and merge the PR. That way, if it breaks on edX.org or in Tutor, we know sooner than later

Relevan ticket: #35173

### Tasks

[farhan]

The implementation PR of this ticket has been reverted because of the issue identified by 2U team.
Issue was notified on the slack channel in thread

Here is the revert PR with details of issue experienced.