TPv2 Fix Miscellaneous issues

[shamrickus]

This PR fixes an issue with the sidebar styling not covering the entire height of the screen and it fixes an issue with the theme-manager.service.ts throwing errors when run in server mode. It also adds functionality for the theme-manager to detect the browsers Prefer Color Scheme setting and will set it to the dark theme if that is preference.

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Which Traffic Control components are affected by this PR?

• Traffic Portal v2

What is the best way to verify this PR?

Open up sidebar entries until it overflows the height, verify that you the actions bar at the bottom covers the sidebar completely

Set your browsers Prefer Color Scheme to dark in the browser settings, clear all local browser data from TPv2, and load the page. It should switch to the dark mode by default.

PR submission checklist

• This PR has tests
• This PR has documentation
• This PR has a CHANGELOG.md entry
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[codecov]

Codecov Report

Merging #7813 (a7e3231) into master (a6b2954) will decrease coverage by 0.01%.
Report is 5 commits behind head on master.
The diff coverage is 81.48%.

@@             Coverage Diff              @@
##             master    #7813      +/-   ##
============================================
- Coverage     65.64%   65.63%   -0.01%     
  Complexity       98       98              
============================================
  Files           323      323              
  Lines         12827    12838      +11     
  Branches        965      970       +5     
============================================
+ Hits           8420     8426       +6     
- Misses         4048     4052       +4     
- Partials        359      360       +1     

Flag	Coverage Δ
traffic_portal_v2	74.35% <81.48%> (-0.05%)	⬇️
unit_tests	74.35% <81.48%> (-0.05%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
.../app/shared/theme-manager/theme-manager.service.ts	86.66% <81.48%> (-5.18%)	⬇️

... and 1 file with indirect coverage changes

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[zrhoffman]

Thanks!