> Why is it an add-on and not just two themes?
I think switching between light and dark themes, should be like a power switch which having on/ off option. Still browser themes haven't bundled with day/ night option. So I had to create an addon instead theme.

> I'd also like to know if there's a way to bring back the 1px lines between tabs.
Actually for me, hiding that line make browser UI more minimal. If you need it back, I think you can use.

.tabbrowser-tab::after,
.tabbrowser-tab::before {
border-left: 1px solid #COLOR !important;
}
---
Now you can find separate themes in,
- https://addons.mozilla.org/en-US/firefox/addon/_night-fury/
- https://addons.mozilla.org/en-US/firefox/addon/_light-fury/