Add white background for no readme container

## What does this MR do?
Changes the readme container background to white so that it is more consistent with the other default pages

## Are there points in the code the reviewer needs to double check?
Check for side effects but there really shouldn't be any

## Why was this MR needed?
Improves UI

## What are the relevant issue numbers?
Closes #20906 

## Screenshots (if relevant)
Before:
![Screen_Shot_2016-08-18_at_4.02.15_PM](/uploads/b3e70bba4c7ecab262bb4ff76bf80d85/Screen_Shot_2016-08-18_at_4.02.15_PM.png)

After:
![Screen_Shot_2016-08-22_at_4.30.41_PM](/uploads/520f276e1ed6786f66e3eff6e579b157/Screen_Shot_2016-08-22_at_4.30.41_PM.png)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5877