Support for themes

[HadrienGardeur]

Our philosophy for themes is based on a continuity between content and reading app. This means that any theme applied to the content will also impact the playground itself in terms of:

• background color
• text color
• primary color
• and secondary color

We'll need to decide if that's the case of fonts too. This can be a little more risky, but in the case of specialized fonts, it could make a lot of sense as well.

Throughout this Epic, we will:

• Set-up support for themes through the use of CSS custom properties
• Implement a default that follows system settings for dark/light modes
• Set up additional themes: light, dark, sepia and high contrast themes
• Add support for theme-switching in user settings

Themes will be a global setting across all publications read by the user, but we might consider differentiating between reflow and FXL though (TBD).