Steps to reproduce:
Precondition: dark or high contrast dark theme

1. Create host with at least 2 host groups
2. Create user that have full access to one of the groups, and read access for the other
3. Log in as the newly created user
4. Open host configuration
5. See the Host groups multiselect field

Result:
The option with no access is too similar to background
See screenshots:

Expected:
Background color of the inaccessible option should be slightly adjusted, so it doesn't look as main background and placeholder.