[CHORE] Refactor chat theme adoption

[filipealva]

@RocketChat/ios

A big thanks to @Sameesunkaria for letting me know that these improvements were needed 👍

[codecov]

Codecov Report

Merging #2267 into feature/integrate-rc-view-controller will increase coverage by 0.02%.
The diff coverage is 100%.

@@                           Coverage Diff                            @@
##           feature/integrate-rc-view-controller    #2267      +/-   ##
========================================================================
+ Coverage                                 25.59%   25.61%   +0.02%     
========================================================================
  Files                                       437      437              
  Lines                                     16513    16507       -6     
========================================================================
+ Hits                                       4227     4229       +2     
+ Misses                                    12286    12278       -8

Impacted Files	Coverage Δ
Rocket.Chat/Theme/ThemeableViews.swift	52.53% <ø> (+1.61%)	⬆️
...Chat/Controllers/Chat/MessagesViewController.swift	44.59% <100%> (+3.26%)	⬆️

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[rafaelks]

🤗