Add support for custom themes

[craxal]

Some users may prefer colors other than what the built-in themes offer. We should provide a way for adding these themes to Storage Explorer.

Custom themes should not require definitions for every color token (we have a lot, and a lot of them are old and need to be replaced). They should have a base theme so that, if the theme itself does not define a color for a token, the value can be pulled from the base theme.