Don't add light mode bg to 3rd and 4th level menu items

[hugovk]

Fixes #5053.

Changes proposed in this pull request:

• Don't add a light mode background colour to fourth-level menu items
• Let them inherit styles instead
• Fixes unreadable menu items in dark mode

Example

https://pillow--5056.org.readthedocs.build/en/5056/reference/ImageFont.html#functions

Dark mode

Light mode

[nulano]

This does fix the readability issue, but the menu looks less uniform without the extra background highlight. Trying the opposite color manually #363636 looks good, although I don't see an obvious way to add it based on the dark/light selection. If that is too complicated then this is obviously worth removing at least for now.

[hugovk]

How's this with CSS? For some reason the RTD PR build hasn't updated dark.css or included light.css in the docs, even though light.css is now on the server.

Anyway, it looks like this locally:

| | |

[nulano]

The screenshots look good.

It's hard to test with the RtD build not updated, but I think this might break Internet Explorer compatibility (since it doesn't support the @media tag). However, I don't think it's worth worrying about a minor style inconsistency on an unsupported browser, so probably fine.