Fix action buttons color matching background color making it not visible in Course theme

[Imran92]

Resolves action buttons color matching background color making it not visible

Proposed Changes

• Made the text colors in the action button have the color of background for course theme.

Testing Instructions

1. Create a course with lessons, make sure learning mode is on
2. Make sure you are on Course theme
3. Try to access the course without logging in
4. Make sure you see the Purchase Course/Sign in buttons in the notice
5. Check for all variations of Course theme
6. Check without Course theme

Before

After

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

[donnapep]

LGTM! Thanks for the find and the fast fix! 🥇

[codecov]

Codecov Report

Merging #7252 (3f85179) into trunk (348fb2b) will decrease coverage by 0.01%.
Report is 48 commits behind head on trunk.
The diff coverage is 44.00%.

@@             Coverage Diff              @@
##              trunk    #7252      +/-   ##
============================================
- Coverage     50.46%   50.46%   -0.01%     
- Complexity    10923    10931       +8     
============================================
  Files           605      605              
  Lines         46027    46062      +35     
  Branches        402      402              
============================================
+ Hits          23228    23244      +16     
- Misses        22472    22491      +19     
  Partials        327      327              

Files	Coverage Δ
includes/class-sensei-quiz.php	69.48% <100.00%> (+0.51%)	⬆️
...ncludes/course-theme/class-sensei-course-theme.php	18.78% <0.00%> (ø)
...es/course-theme/class-sensei-course-theme-quiz.php	0.00% <0.00%> (ø)

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