Theme options, save theme preference and oscdark enhancements

[pavlidvg]

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Adds support for switching back to the System Default after enabling dark mode, as well as saving the user preference for the next startup.

2 small changes were also made to the dark theme, for improved readability. Links are now a light blue shade (similar to discord or JetBrains links on dark mode). The 'Download' dialog is now correctly dark.

A few new code snippets are missing comments. It is because I am already working on a pull request that slightly changes them and adds proper documentation.

[pavlidvg]

New commits should correctly implement theme file loading on dev and frozen, using resource_path correctly. Also, @dhtdht020 was right, the program path join was unnecessary, I previously misunderstood the resource_path function

[dhtdht020]

Sorry for the long wait, will look at this again in the coming days.