[02] Refactoring code (Reopening of PR #2469, plus some modifications to make PR #2629 merge easily without any problem)

[og-mrk]

More Details on changes?

This PR is basically the same as PR #2469, but with the following simple changes:

• Had to revert simple changes in Get-TabXaml function, as this function will be removed by @MyDrift-user 's Refactoring PR, making the merging process for both PRs alot better. ( commit 4a89626 )
• @MyDrift-user and I have agreed on a solution to fix the upcoming Merge Conflicts related to themes.json File.. which's transferring some of his code changes into this PR.. I made sure to give proper credit to these changes by making him the commit author of those changes. ( commit 591675e )
• other then that.. nothing else, see more details about changes in the next section(s).

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Title

Second PR in UI Refactoring Series.

Type of Change

• New feature
• Refactoring
• UI/UX improvement

Description

Implements the Default Theming Idea I had when I opened issue #2226, and although I must say that this PR is alot smaller in code changes / Impact on End User.. but I do think it's a something overlooked, and can be improved upon (there's still room for improvement even after this PR).

Testing

WinUtil compiles & runs successfully, and the theming looks correct (as far as I can tell).

Impact

End user won't feel any difference, but when adding or updating theming options across different themes.. this can be done with ease by simply changing the _default theme.

Additional Information

I haven't tested startup time difference before & after changes, but due to the increased error checking & other changes as well.. I won't be surprised if it's increased by a small margin.

Checklist

• My code adheres to the coding and style guidelines of the project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• I have made corresponding changes to the documentation.
• My changes generate no errors/warnings/merge conflicts.