Add a color scheme toggle to the header bar

[daanboer]

Also implements more uniform styling across the application.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.26%. Comparing base (5dc90d9) to head (4f8fe57).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1324   +/-   ##
=======================================
  Coverage   81.25%   81.26%           
=======================================
  Files         233      234    +1     
  Lines        6204     6206    +2     
  Branches     1077     1077           
=======================================
+ Hits         5041     5043    +2     
  Misses       1120     1120           
  Partials       43       43           

Flag	Coverage Δ
app	57.45% <100.00%> (+0.08%)	⬆️
converter	65.31% <ø> (ø)
database	83.56% <ø> (ø)
schema	94.15% <ø> (ø)

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