bug: fix selected text bg colour being wrong if only the fg colour was set

[ClementTsang]

Description

A description of the change, what it does, and why it was made. If relevant (such as any change that modifies the UI), please provide screenshots of the changes:

Fix selected text background colour being wrong if only the foreground colour was set, due to conflicting colour usage.

This is resolved by just using a single source of truth for the default selected text style. This PR also cleans up some duplicate colour code, and adds a test that originally failed.

Issue

If applicable, what issue does this address?

Closes: #1020

Testing

If relevant, please state how this was tested. All changes must be tested to work:

If this is a code change, please also indicate which platforms were tested:

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[codecov]

Codecov Report

Base: 20.73% // Head: 20.96% // Increases project coverage by +0.23% 🎉

Coverage data is based on head (457f68f) compared to base (a266dd7).
Patch coverage: 66.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1021      +/-   ##
==========================================
+ Coverage   20.73%   20.96%   +0.23%     
==========================================
  Files          75       75              
  Lines       14683    14694      +11     
==========================================
+ Hits         3044     3081      +37     
+ Misses      11639    11613      -26     

Impacted Files	Coverage Δ
src/bin/main.rs	36.58% <ø> (ø)
src/canvas.rs	0.00% <ø> (ø)
src/components/data_table/styling.rs	9.09% <ø> (ø)
src/options.rs	41.94% <ø> (ø)
src/widgets/cpu_graph.rs	0.00% <ø> (ø)
src/widgets/disk_table.rs	0.00% <ø> (ø)
src/widgets/process_table.rs	14.35% <ø> (ø)
src/widgets/temperature_table.rs	0.00% <ø> (ø)
src/canvas/canvas_styling.rs	45.80% <63.63%> (ø)
src/canvas/canvas_styling/colour_utils.rs	93.70% <100.00%> (ø)
... and 2 more

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