add theme settings onto library's settings menu

[ana-rchy]

Is your feature request related to a problem? Please describe.
i just spent an hour trying to mess around with gtk settings and environment variables to no avail to make foliate dark themed, only to realise you have to go into a book first and then click on the three dots to change theme

Describe the solution you'd like
show theme settings in the library's settings menu

Describe alternatives you've considered
N/A

Additional context
N/A

[SuzumiyaCC]

Theme menu will bi nice!
Please add it to you app