Beta 14

[JayPanoz]

This PR aims to make Readium CSS more consistent following implementers’ feedback.

Primarily, this:

• consolidates custom colors for theming into a single user submodule;
• removes the fontOverride flag following the removal of advancedSettings;
• removes day mode and migrate its declarations in base
• creates opt-out flag for overflow

[JayPanoz]

This is now complete.

Overflow complications will be handled in another version, TBD.