The colors of the correct/incorrect banner for quizzes are not customizable

[s-kennedy]

Describe the bug
There's a banner that pops up at the bottom of the quiz in the correct/incorrect quiz mode that tells you if your answer was correct or incorrect, and it has a green or red background. However those colors are not customizable, even with custom CSS, it's just hard coded into the markup, right here.

Expected behavior
I'd like to be able to style that banner, either through the theme or through custom CSS. I'm happy to open a PR if this is a feature you want to include.

[salmanlakhani-dev]

Hey @s-kennedy
can you help me with this
#174