UI/UX: Add styling for fill-in-the-blanks text

[Mochitto]

As of now, since there is no style defined by the themes, the colors used by the addon are green, orange and red.
These work when clozes' colors have good contrast with the addon's, but doesn't for the current themes.

There should be extra styling to make sure the contrast of the addon's text works out well with the rest of the theme.
Notice: the addon already shows the "wrong text" if there are mistakes, so maybe using just one color could work well; if that's the solution, then all it takes is making sure the clozes' color is never overwritten (!important).

[Mochitto]

You can understand how to navigate the project and find where to add the style in CONTRIBUTING.md.

Extra styling rules should be added to themes to customize the look of clozes.