Scope CSS to Learning Mode templates only

[donnapep]

Resolves #7226.

Proposed Changes

Some Learning Mode CSS is affecting non-Learning Mode templates. Given that it's not possible to conditionally load CSS for some templates but not for others, this PR scopes the CSS better. This problem was introduced by 997bbf8. Although I tried to reproduce, I didn't see any issue with Astra in the editor after reverting that change.

Having said that, I think an argument could be made as to whether we should be so opinionated about our CSS in the first place, especially for block themes. In the future, we may want to consider defaulting to the theme's fonts, colors etc., and just provide sensible defaults to classic themes (or maybe not even for classic themes either 🤷🏻‍♀️ ).

Testing Instructions

1. See Sensei overrides block theme's default styles #7226 to ensure it's fixed for block themes.
2. Activate Astra.
3. Check the Lesson and Quiz Learning Mode templates. They should look the same or similar as before.
4. Repeat for Divi or any other classic theme.

Pre-Merge Checklist

• PR title and description contain sufficient detail and accurately describe the changes
• Acceptance criteria is met
• Decisions are publicly documented
• Adheres to coding standards (PHP, JavaScript, CSS, HTML)
• All strings are translatable (without concatenation, handles plurals)
• Follows our naming conventions (P6rkRX-4oA-p2)
• Hooks (p6rkRX-1uS-p2) and functions are documented
• New UIs are responsive and use a mobile-first approach
• New UIs match the designs
• Different user privileges (admin, teacher, subscriber) are tested as appropriate
• Code is tested on the minimum supported PHP and WordPress versions
• User interface changes have been tested on the latest versions of Chrome, Firefox and Safari
• "Needs Documentation" label is added if this change requires updates to documentation
• Known issues are created as new GitHub issues

[codecov]

Codecov Report

Merging #7382 (934fb41) into trunk (b20234f) will increase coverage by 0.01%.
Report is 5 commits behind head on trunk.
The diff coverage is 0.00%.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk    #7382      +/-   ##
============================================
+ Coverage     50.75%   50.77%   +0.01%     
+ Complexity    11149    11145       -4     
============================================
  Files           613      613              
  Lines         47057    47042      -15     
  Branches        404      404              
============================================
  Hits          23884    23884              
+ Misses        22846    22831      -15     
  Partials        327      327              

Files	Coverage Δ
includes/class-sensei-settings.php	57.85% <0.00%> (+0.88%)	⬆️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 844675b...934fb41. Read the comment docs.