fix: scope global sidebar styles to the app

[st3iny]

How to reproduce?

1. Install Talk.
2. Open a chat room and open the right sidebar.
3. Observe the header of the right sidebar.

[st3iny]

/backport to stable4.7

[Antreesy]

Tested, back to normal:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 29.29%. Comparing base (c01de6e) to head (2460c53).
Report is 11 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6081   +/-   ##
=========================================
  Coverage     29.29%   29.29%           
  Complexity      914      914           
=========================================
  Files           287      287           
  Lines         13987    13987           
  Branches       2139     2150   +11     
=========================================
  Hits           4098     4098           
+ Misses         9575     9574    -1     
- Partials        314      315    +1     

Flag	Coverage Δ
javascript	15.18% <ø> (ø)
php	58.65% <ø> (ø)

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