The back button for the Monokai and Gruvbox themes are black on a dark background, making it nearly invisible.
For other themes, the back button matches the primary colour colour of the theme (i.e. green for Monokai), which would be visible on their backgrounds. Should be a quick css fix (which I would love to try tackling!)
Monokai:

Nord (for comparison):

The back button for the Monokai and Gruvbox themes are black on a dark background, making it nearly invisible.
For other themes, the back button matches the primary colour colour of the theme (i.e. green for Monokai), which would be visible on their backgrounds. Should be a quick css fix (which I would love to try tackling!)
Monokai:

Nord (for comparison):
