Add light and dark as valid values for set-local-storage-item

[partingscientist]

Considering My Filters is now untrusted by default and setting site themes is one of the original use case presented for introducing set-local-storage-item in #2697, I think the values light and dark would be nice additions to set-local-storage-item for users using permanent incognito mode/private browsing.

Several use cases for considerations:

1. Setting light theme for FotMob (fotmob.com) currently requires using
   fotmob.com##+js(trusted-set-local-storage-item, mode, '"light"')
2. Setting dark theme for Kaggle notebooks (kaggle.com) currently requires using
   kaggle.com##+js(trusted-set-local-storage-item, KAGGLE_THEME, dark)
3. Setting dark theme for ChatGPT (chat.openai.com) currently requires using
   chat.openai.com##+js(trusted-set-local-storage-item, theme, dark)

[gorhill]

We will have to put a stop on requests to support more values, otherwise what's the limit? Might as well import words.txt? Given the specified use case, just use trusted-set-local-storage-item, what you are proposing is not going to be used in a filter list, it's a case of personal preference.