Add dark theme

[CompSciOrBust]

On Switch this will automatically be applied depending on the system theme. On other platforms (without dark theme support) a method to set isDarkMode in ThemeManager.hpp is needed. I can add a config file for this if you want, but if I'm going to do that maybe I should add support for loading custom themes from the config too?

[vgmoose]

Thank you so much for this! And sorry for taking so long to get to it.

My only issue as-is surrounds the way Buttons work in Chesto (it treats "dark" themed buttons as the highlight color that's used by hb-appstore, and so they light theme buttons here seem out of place). These button colors should instead be specifiable through the Chesto's API.

As for hb-appstore on its own though, this is a great overhaul to the hardcode light theme constants and I want to merge it sooner rather than later, as it's easier to work with for future changes.

I also think that the AppCards should probably have a more defined background, rather than just floating out in the Ether of the AppList, but that feedback applies to both light and dark themes.

Regarding a config file, I'd consider that another separate concern. I'm picturing the following as configurable preferences:

• Theme color (Auto (Based on system config), Light, Dark)
• Accents color (Auto (Red on Switch, Blue on Wii U), Red, Blue, other colors)
• Volume slider and Mute state
• Display resolution and text scaling

As well as a UI to adjust the above. There's not really much in place for that yet either, although I would imagine it'd be a json file that lives in the hb-appstore folder, relative to the wuhb/nro.