To stop the menu links from appearing side by side on tablet and below:
-select the tablet viewport
-double click on the header, then use the open menu button in the settings tab
-select a nav link, and shut off "float left" (as shown below, notice the float: none; that I set)
Explanation: When float is on all elements will only take up as much room as they need rather than the entire width. So you want float: left on for desktop view so they all appear in one line, but on mobile displays shutting float: off will cause each menu link to appear on their own line.
PS: you can also remove the 16 pixels of margin on the bottom of the menu links on mobile to get rid of that whitespace.
Hope this helps