This addon will show you the current spells you can cast based on the modifier keys you're holding down.

/ctctt opens the options panel for the addon which is also available under Esc->Options->AddOns->Click-To-Cast Tooltip:

Here you can enable or disable top/bottom dashed/empty lines as well as set the color for the button name or the spell name.

This is an example tooltip and how it looks with ElvUI:

This has all the options enabled as well as the default colors of red for the button and green for the spell. Check out the image gallery for further examples.