Aviator: How can I underline an active page in the navigation?

Working in Aviator and I know this is not standard functionality but it must be possible with CSS. I want a line under the active page in the navigation instead of it being a different colour. How do I achieve this?

Use this in your Custom CSS, changing YOUR-LINK-COLOR with your desired color.

#main-navigation ul li.active-link>a{
   text-decoration:underline;
   color:YOUR-LINK-COLOR;
}

Thank you Ghostcat that works! How can I however make the line appear a little bit lower, so have a bit of spacing between the text and the underline? Also I noticed that it does only appear once you have clicked on an element so not when you first land on a page. Is that fixable?

You can't change the spacing of the text-decoration underline since that is determined by the font. However, you can use a 1px bottom border instead like so:

#main-navigation ul li.active-link>a{
   border-bottom: 1px solid YOUR-LINK-COLOR;
   color:YOUR-LINK-COLOR;
   padding-bottom:1px;
}