Change navigation font color on mobile sizes

I would not modify the theme css file. It may be replaced if the theme is updated. I would instead create a custom css file that is listed below the theme css file. The style that would override the color of the mobile menu text is:

.genesis-nav-menu.responsive-menu .menu-item > a:hover {
    color: #fff;
}

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Yes, sorry, I meant I would use simple custom css plugin.  Your suggestion worked for the main navigation buttons on mobile device but the submenus don't display any change at all.  is there a way to add the blue background with white text to submenus too?  If you want me to create a new question, I will.  Thanks

Those items currently have a transparent background color. The selector for the style that targets them is

.genesis-nav-menu.responsive-menu .sub-menu li a, .genesis-nav-menu.responsive-menu .sub-menu li a:hover

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Thanks.  I tried adding a background color to the above selector but it didn't appear to take.

.genesis-nav-menu.responsive-menu .sub-menu li a, .genesis-nav-menu.responsive-menu .sub-menu li a:hover {
  background-color:#0088ce;
  color:#fff;}

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Any idea what I did wrong?

Perhaps I misunderstood. Are the submenus displayed when you click the down arrow at the end of the main menu item? Those items have changed color and background-color.

I was having a caching issue.  So now, yes, ALL the submenus appear as light blue.  Not really what I was looking for so I will have to see if I can tweak.  Give me a moment.

Cool - OK got it to work almost perfectly.  Trying now to remove the dark blue hover on current page on sub menus.  Can't seem to find the code to remove that.  It is hard to use Inspect Element to view code because drop down menus go away.