Consistency of ReadiumCSS filters across Themes

[JayPanoz]

When using ReadiumCSS sepia mode/theme, you get the blending of images out of the box. Currently, paper and contrast3 (a.k.a. mint) don’t do that, which is inconsistent.

Since these filters are now exposed globally through the latest PR, this means you can reuse them as you see fit, although you would have to do that manually in code based on the theme’s name cause we don’t have anything to cover that in theme properties in preferences.

There are 2 ways to see this issue, which are not exclusive of each other:

1. Should ReadiumCSS’ sepia mode/theme blend images by default? We’ve already had an instance where this was an issue.
2. Should we extend theme props in preferences so that you can define images are blended, darkened, and/or inverted (+ gaiji)?

Notes:

• the darken and invert filters now accepts percentages in addition to the readium-{filter}-on keyword/flag that was previously implemented so that you can try to match the color of your custom background;
• there is the gaiji case for invert on top of that, applying to Japanese books only.

Any opinion? I know we discussed this with @HadrienGardeur yesterday and @danielweck opened the issue I referred to above so they should be interested in this issue but any feedback obviously welcome.