Deprecate isDarkMode in favor of useThemeStore

[srukelman]

Summary

Refactored implementations of isDarkMode() (from helpers) in favor of useThemeStore

Closes #812 #854

[MinhxNguyen7]

This mostly looks good other than a different change that slipped into the commit. Also, instead of deprecating isDarkMode() and, therefore, leaving it there, just delete it since nothing uses it.

Because I'm a pedant, I'll say that the the better implementation of appTheme == "dark" would be appTheme.isDark() so that "dark" and "light" aren't magic values. Intellisense tells you, but you ideally shouldn't need Intellisence. I know you didn't write the interface to begin with, so I won't make you do the search-and-replace (but it should be pretty easy). I say it for the sake of future code.

Also, for the future, please fix the merge conflicts so that there is at least one deployment so the reviewer doesn't need to spin up a local instance.

[MinhxNguyen7]

Sorry for holding your PR up in review. Everything looks good except for the logic I mentioned. I can't let that slide.

[MinhxNguyen7]

LGTM!