You target the hover action with the :hover style. On my test site with one of the built in templates, there's a gradient on the btn elements, so you may need a line like the second one for background image - You may or may not need this second style for your template.
.scuser-menu a.dropdown-toggle:hover {
background-color: YOURCOLOR;
background-image: linear-gradient(to bottom,#fff,YOURCOLOR);
}
If you're in Chrome or a similar browser, there are developer tools where you can inspect the elements on the page. If you select a regular button, you should be able to see what CSS is being applied. Once selected, then you can also select the :hover attribute. It helps to make sure you copy the same color values as other pieces on your website.
-Melissa