Did you play around with ACP>Appearance>Styles>yourstylename>Header and Navigation>Navigation Tabs Container / Navigation Tab / Selected Tab? I don't know about a true round button, but an oval button should be easy.
Change the container background to match your header color and the tabs will look to be "all alone". Then change the tab background color to whatever and add a border with a radius equal to half the height and that will make an oval button. Style the text color for a good contrast.
I won't go into details but your going to need to make some changes in the style properties, add some changes to the extra.css template (because there are some areas in that styles navigation/dropdown that are not supported added in the SP) and you will also need to be fluent with designing in a graphics editor because that navbar is all imagery from the selected, hovers to the maintab and tablinks.
I'm quite sure a template edit will be needed since the .visitorTabs area have been moved out of the navigation area and moved up by the moderatorBar area.