Docs: Clarify CSS variable usage scope (Fixes #3951)

[Hari-varshan]

Fixes #3951

Summary

Clarified the documentation regarding CSS variables. While variables are globally defined in the Theme, this update adds a note explaining that their usage follows standard CSS inheritance rules:

• Values are inherited by descendants (waterfall).
• Values are isolated from siblings.

Note: Skipped CHANGELOG.md update as this is a documentation-only clarification.

[TomJGooding]

I don't think this is the 'variable scope' referred to in the issue:

For example, it would seem that if you're using CSS_PATH, you can't define a series of variables in one file earlier in the path, and then use those variables in files later on in the path.

Explaining basic CSS rules seems pretty redundant. But I strongly suspect this PR was just generated with AI.

[Hari-varshan]

Hi, thanks for your feedback. I'm new to contributions. I picked this issue because I felt it was easy and thought the requirement was just a basic CSS rule. Yes, I used AI to draft the sentences for the documentation.

After reading your comment carefully, I realized the issue is about file scope. I ran some local tests and confirmed that CSS_PATH does not act as a collective store that inherits variables from one file to the next; instead, it isolates the file variables separately.

Is my understanding correct now? Should I close this PR and open a new one?