Setting SfComboBox's Background color does not affect the dropdown's background, but setting text color does affect dropdown text color. This makes for an impossible situation in dark mode. Either dropdown color should match the entry box's or both the dropdown's background and text colors should be independent of other settings.
Dropdown height ought to be no more than the sum of its visible items and the control ought to be smart enough to assure that, unless a control template is provided. While the dropdown height is programmable, that should only be required if a full list is going to be too tall.