I used a vSphere Center 6.7 build 14368073 with the dark theme. As you can see in the capture button to move up and down the uplink in the distributed port group are invisible. No problem with the standard theme.
It is like it is
"vSphere Client plugins (including VMware and Partner plugins) are still being upgraded to the new dark theme. Expect some plugins to exhibit visual inconsistencies."
You will not change it. Maybe next release or patch will improve that.