You should not overwrite css in MUI's theme file, the problem you encounter is because many MUI components use the MuiList component (There are many basic components that are reused in complex components such as Drawer, Table, ...).

You also cannot css through class from sx or makeStyles because the dropdown of select is not a child component inside the root select but it creates a node outside the body tag (equal to the root if using react).

There are 2 ways to handle droppdown styles:

• styled() utility: this way helps us create a completely new component based on an existing MUI component and write the css type. The advantage is that we can reuse this new component in different screens.
• Through MenuProps: usually for components that are displayed outside the body (portal style), MUi components often provide props to handle that, for example in the select component, it is the MenuProps props