feat(appearance): a Black theme for AMOLED screens

Adds Black next to Light and Dark in the theme picker: the dark theme
with pure black backgrounds, applied wherever the app theme is used.
Moves floret-kit to main, which brings ThemeMode.BLACK and the theme's
pureBlack, and has the submodule track main instead of feat/clockula.

.gitmodules

@@ -1,4 +1,4 @@
 [submodule "floret-kit"]
 	path = floret-kit
 	url = https://codeberg.org/jlmakiola/floret-kit.git
-	branch = feat/clockula
+	branch = main