In the file en/help.md, we read (for setting a theme):
(<kbd>⌘</kbd>+<kbd>,</kbd> on Mac)
(In the following, I write cmd instead of ⌘ as the later is perhaps not displayed in all operating system; it's the command key)
This don't work on the Safari browser: the shortcut cmd+, is the standard shortcut of a Mac application for its preferences (settings).
If I press cmd+, the window of Safari preferences is opened.