bug: dark theme selected dropdown colour turns white

**Describe the bug**

When an option is selected on the dark theme dropdown, and other options are being hovered, the selected option's colour updates to white, making the selected option unreadable.

**To Reproduce**

Steps to reproduce the behavior:

1. Use Windows/Ubuntu to recreate this bug. Mac will show the default OS dropdown stylings.
2. Go to [dark forms](https://vanillaframework.io/docs/examples/patterns/forms/forms-dark).
3. Select an Ubuntu version from the dropdown
4. Hover over other options.
5. See the error - selected option should turn white.

**Expected behavior**
The selected option should change to a different background colour, making the option readable.

**Screenshots**

**Desktop (please complete the following information):**

- OS: Windows, Ubuntu Jammy
- Browser Microsoft Edge, Chromium
- Version: 119.0.2151.72

Start at the dark forms example linked in the issue and inspect the dropdown styling used there. Reproduce it on Windows or Ubuntu in Chromium or Edge by selecting an option and hovering over others; done means the selected option remains readable with a distinct background colour.

Written by the indexing model from the issue text.