[Docs] Search result list items have light background in dark mode

**Describe the bug**

**Impact and severity**

Low severity. No workaround. It's a bug in our documentation website.

**To Reproduce**

1. Go to https://eui.elastic.co/.
2. Switch to dark mode.
3. Write "doc" in the search bar.
4. Notice how the hovered item has white background.

**Expected behavior**

The background color is adjusts to color mode.

**Screenshots (Optional)**