"Use current browser theme" doesn't follow changes to "System theme"
In Firefox 89 there is a "System theme" that syncs up with the OS's theme, e.g. on macOS which can be "Auto", meaning light mode in daytime and dark mode at night.
Tab Center Reborn doesn't seem to respect the Firefox System theme when it is equivalent to the Dark theme: it looks (nearly) identical to when the Light theme is activated.
Since the whole point of the System theme is that it changes dynamically, this issue can't really be worked around with custom CSS.
Edited by Aaron Madlon-Kay