div.mx-navigationtree div.navbar-inner ul li a[role="menuitem"]:hover{
background-color: whatever-color-you-want
}
But it is better to use the variable that Mendix has made available for you. To find that variable, go to this file "\themesource\atlas_core\web\core\widgets\_navigation-bar.scss" and you will see
You can find the current value of $navigation-bg-hover by looking for it in \themesource\atlas_core\web\_variables.scss
// Default Navigation styling
$navigation-bg: $brand-inverse !default;
$navigation-bg-hover: lighten($navigation-bg, 4) !default;
$navigation-bg-active: lighten($navigation-bg, 8) !default;
To change that value of navigation-bg-hover, see the text in the header of that file:
// DISCLAIMER:
// Do not change this file because it is core styling.
// Customizing core files will make updating Atlas much more difficult in the future.
// To customize any core styling, copy the part you want to customize to styles/web/sass/app/ so the core styling is overwritten.
Meaning: if you want to change navigation-bg-hover to something other than "lighten($navigation-bg, 4) !default;", you need to do so in a separate file _custom.scss (or if you want :hover to differ from :focus and :active)
.navigation-bg:hover
{
background-color: blue
}
and include _custom.scss in main.scss by adding:
@import "custom";
Actually if you are fine with navigation-bg-hover being just a lighter version of navigation-bg, you only have to change the value of navigation-bg in a file called custom-variables.scss and include that in main.scss.