fix: ora grading iframe dark frame issue

[HammadYousaf01]

Fixed the dark theme issue within ora grading. The only solution I could come up with was to change the background color instead of the text color. The issue was that an iframe within an iframe didn't allow us to target styles.

Fixes https://github.com/overhangio/tutor-indigo/issues/120

[DawoudSheraz]

@HammadYousaf01 Please add appropriate screenshots.

[Danyal-Faheem]

The current response editor shows up with a black background during dark theme but the font is also black which makes readability very difficult.

We now change the background to a light gray colour instead. Ideally, we would change the font to a white colour for consistency with the rest of the theme however, the response editor is an iframe embedded inside another iframe. Due to browser security controls, we cannot access the DOM content of these iframes and thus cannot modify the text colour.

Here is an example of how it looks now:

This new colour is also WCAG compliant with our background dark colours.