Wiki TOC background color dark theme

The background color of the wiki TOC is light grey, regardless of the theme.

That makes the text within the TOC barely readable when the dark theme is applied.

Steps to reproduce:

1. Open the moodle app, log in to your account at a moodle site.
2. Go to the app settings and change the theme to dark.
3. Navigate to a course and open or create a wiki within that course
4. Open or create an article within that wiki that has multiple sections.

Expected behavior:

The TOC background color should be darker, so that the light text is readable.