While nitpicking I found that vertical rhythm is broken by %tabs. Thats because borders are being set, while not taking that fact into account when setting the margins. For my own purpose I came up with a solution, albeit not the most elegant one. In the file STARKIT/sass/_modular-styles.scss on line 286 find:
margin: rhythm(1) 0;
I changed this to:
margin: rhythm(1, $offset: 2px) 0 rhythm(1, $offset: 1px) 0;
I have not been able to test what happens with secondary tabs, they may need a different margin. For primary this works because at the top 2 borders of 1px are being set, to the bottom 1 border of 1 px.
I just wanted to make a note of it as it seems to be a simple fix.