Update Toolbar icons, and add theming support for the context menus

Is your feature request related to a problem? Please describe.

As you can see, the context menu is not in dark mode. The toolbar icons in Saucedacity/Tenacity are too big and make it seem less 'professional'.

Describe the solution you'd like
Implement a dark mode like the Notepad++ approach.

Still in awe how they got an outdated, old-school looking program into something as nice as that.

Describe alternatives you've considered
I would've considered Audacium, but it looks like it's also dying and the last update was months ago.

[generic-pers0n]

@n0toose made a POC about setting the appropriate theme (light or dark) on Windows. I am currently waiting to make a similar POC after everything is settled and taken care of, but right now, that is currently on hold, though hopefully for not much longer.