Feature: Add theming functionality

[Lamparter]

Description and Motivation

Adds theming to FluentHub. This can be achieved by creating multiple files with different Primer resources that the user can select.

Motivation & Context

Related: #158
Related: #163

### Tasks
- [ ] Improve border and some of SolidColorBrushes which contains in FluentHubDefault.xaml theme resources to use system default theme resources.
- [ ] Add theme switching helper class methods
- [ ] Add various GitHub's official themes
- [ ] Display those themes in app appearance settings page

[0x5bfa]

I copied and converted(argb -> hex) from here https://primer.style/primitives/colors

And to achieve this goal, I think we have to create theme switcher, of course.