Replying to my own comment: HN faux pas, or good information organization? :)

In the Tree Style Tab options page, there's an Advanced section that has a live-reloading user style sheet section. Very cool for testing out font choices without restarting the browser.

I've changed mine to use Apple's really nice SF Pro font, condensed. Somehow the Iosevka Mono that I use everywhere didn't look quite right on the tab titles.

The CSS:

  :root.sidebar tab-item.unread .label-content {
    font-style: italic !important;
  }

  :root.sidebar tab-item {
    font-family: "SF Pro" !important;
    font-stretch: condensed !important;
    font-weight: 300 !important;
  }

  :root.sidebar tab-item.active .label-content {
    font-weight: 500 !important;
  }

Same link as my previous comment now has a screenshot of the result of that CSS: https://gist.github.com/aclarknexient/88673880d373864eee1927...