To add to Mehmet's answer: if you want to find out which classes to use for a particular part of your page, the easiest way to find out is to use your developer tools in your browser.
For instance, say I want to find out which classes I need to target the “comment” button here in the thread, I open the developer tools (I'm using Chrome, where the shortcut for that is F12). I then click the “select element” button or press Ctrl + Shift + C to select the element I want to target:
Now, click the element you want to learn the classes of:
On the right side, you can see all the classes on this button. You'll be able to use these for styling. You do need to keep in mind that there is such a thing as Specificity in css – the more specific the selector, the higher the priority of the styling you put in there. This means you might need to add a bigger selector to overrule some of the existing styling on the menu.
While this doesn't answer your question directly with regards to the menu, this is a method you can use to figure out the classes for your styling in general and you'll be able to use it almost everywhere, so it might help you in the long term.
You can find detailed information in this link. https://forum.mendix.com/link/questions/109963 The answer to the question is explained in detail.
Good luck with Mendix
Go to your project folder (in modeler via … Toolbar [App] > Show App Directory in Explorer )
Than go to theme > web > custom vaiables.css. Here you will find your options in order to change the hover color.