Add color themes to reader settings popup

[dstillman]

As on desktop, to allow switching between light mode and dark mode regardless of system setting

We should also add support for Sepia, which exists in PDF Viewer and is now supported in the EPUB reader. (We may have to leave out "Snow", since — assuming they don't allow custom color schemes — there's no equivalent in the PDF reader.)

[michalrentka]

As on desktop, to allow switching between light mode and dark mode regardless of system setting

We allow that on iOS too

We should also add support for Sepia, which exists in PDF Viewer and is now supported in the EPUB reader. (We may have to leave out "Snow", since — assuming they don't allow custom color schemes — there's no equivalent in the PDF reader.)

Added sepia in #1070, custom schemes or backgrounds are not supported. I could contact their support and see their reaction.

[dstillman]

We allow that on iOS too

Oh, whoops, I totally missed that last option.