The selected entry in Gmail's menus and auto-completion lists gains an obvious highlight, giving you good feedback when you use the arrow keys or mouse to navigate these lists.

Works for me on Chrome 40 and Firefox 35 on Windows using Stylish. If at first it doesn't work for you, try reloading Gmail, then try restarting your browser, then leave relevant details in a comment here and I'll do my best to fix it.

Highlights selected entry in:

* Gmail, Contacts, and Chat search
* Gmail Composer To/Cc/Bcc addresses
* every menu I could find

If you find somewhere that doesn't get highlighted, please comment here and I'll do my best to add it!

The highlight color is an obvious but not glaring blue. (Gmail's default highlight is virtually invisible on my screen.) You can make a simple edit to the CSS code in this userstyle to choose your own highlight color. The CSS code ("Show code" button below) includes documentation of which class selectors affect which elements.