We recently had an error that would've been caught by type checking in the frontend (see #2393). This PR implements type checking so that future errors will be caught.
Additionally, this PR contains miscellaneous frontend cleanup that's tangentially related to type-checking.
The current method for checking light vs. dark mode is to compare the
name of the theme against a hardcoded string. This PR uses a dark mode
boolean. This change is for verbosity and so that we're not reliant on
theme name. Additionally, we change some references to colors to the
global theme, instead of indexing the list of themes.
