User directory: Insufficient color contrast for User directory sync messages

Issue Summary

Due to the insufficient colour contrast on the Dark theme, the user directory sync messages are not easily readable.

This is reproducible on the Data Center: yes

Steps to Reproduce

1. Enable Dark theme.
2. Navigate to Administration > User Directories.
   Make sure to have an LDAP connected.
3. Click on _ Synchronise_.

Expected Results

The sync messages are readable.

Actual Results

The contrast looks good on the Light theme:

Workaround

1. No workaround is available besides switching the theme back to the Light theme.