Top: EuiComboBox. Bottom: EuiSelect.

The drop down arrow disappears for the EuiSelect, but not the EuiComboBox, and also the border color of the EuiComboBox is noticeably brighter (in dark mode).

Here's a simple sandbox that reproduces it: https://codesandbox.io/s/gifted-flower-r7z81n?file=/demo.tsx