Add success message styles

[renatho]

Based on #7204

Proposed Changes

• It just adds success styles to the sensei messages.

Testing Instructions

See https://github.com/Automattic/sensei-pro/pull/2435

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 #7209 (524436f) into add/styles-for-sensei-login-page (c4a2040) will increase coverage by 0.03%.
The diff coverage is n/a.

@@                          Coverage Diff                           @@
##             add/styles-for-sensei-login-page    #7209      +/-   ##
======================================================================
+ Coverage                               50.03%   50.06%   +0.03%     
- Complexity                              10705    10721      +16     
======================================================================
  Files                                     601      601              
  Lines                                   45200    45231      +31     
  Branches                                  402      402              
======================================================================
+ Hits                                    22616    22647      +31     
  Misses                                  22257    22257              
  Partials                                  327      327              

see 4 files with indirect coverage changes

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[Imran92]

LGTM 👍