chore: Converts builtin block Sass variables into CSS variables

[farhan]

Ticket: #35306

chore: Converts builtin block Sass variables into CSS variables

Description

Describe what this pull request changes, and why. Include implications for people using this change.
Design decisions and their rationales should be documented in the repo (docstring / ADR), per
OEP-19, and can be
linked here.

Useful information to include:

• Which edX user roles will this change impact? Common user roles are "Learner", "Course Author",
  "Developer", and "Operator".
• Include screenshots for changes to the UI (ideally, both "before" and "after" screenshots, if applicable).
• Provide links to the description of corresponding configuration changes. Remember to correctly annotate these
  changes.

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Other information

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• If your database migration can't be rolled back easily.

[farhan]

Closed as created new one: #35385