Update IDE theming to use CSS variables

What does this MR do?

Issue: #214702 (closed)

By using CSS variables, we can make creation of themes for the Web IDE a much easier process, while also reducing the size of the resulting CSS bundle. (The CSS bundle won't need to increase in size by several folds when adding a new theme, and would just need a change in variables.)

Screenshots

Have manually verified that this refactor does not cause any UI change.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team