Campaigns beta message box has a white background in dark mode

Created by: Joelkw

Steps to reproduce:

1. Make sure your theme is in dark mode
2. Visit [your private sourcegraph domain]/campaigns, like https://sourcegraph.test:3443/campaigns

Expected behavior:

You would see a dark-background box, like the "create your own extensions" box on the extensions page in dark mode.

Actual behavior:

We see a white-background box regardless of theme.