Steps to reproduce:

1. Use Firefox-engine based browser(Firefox - 66.0.3 (64-bit) , Waterfox - 56.2.8 (64-bit))
2. Open Zabbix Web GUI
3. Switch to dark color theme
4. Navigate to Configuration -> Host
5. Open items and create/edit item.
6. Navigate to Preprocessing and click on add new
7. Click on drop-down menu

Result:
See screenshot attached. The section names are "whited"-out

Expected:
Section names must be readable.