fix(theming): fix CSS with nested rules parsing when registering a theme

[Splaktar]

PR Checklist

Please check your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
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PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

• the regex used to split the CSS string breaks nested media query rules

Fixes #9869. Closes #9872.

What is the new behavior?

• replace the regex used to split the CSS string with a function that takes into account nested rules
• remove unused ruleMatchRegex

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

This PR continues the great work by @natete in PR #9872, which was thoroughly reviewed. This PR just rebases, resolves conflicts, and makes some minor formatting and naming changes.

You can see screenshots of how this is broken and how the correct rendering should look (which this PR achieves) in #9869 (comment).

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[Splaktar]

@natete can you please add your consent here as requested in the previous comment? ¡Gracias!

[Splaktar]

Consent posted over in the old PR here: #9872 (comment)