mmh, that's weird - for me, on macOS Sierra 10.12.6, with TheBrain 9.0.227, neither Alt-Tab, nor Alt-Shift-Tab, nor Alt-Ctrl-Tab, nor other keyboard shortcuts works as described for 'Next Content Tab'. To be sure, I've restarted TB each time when I changed the keyboard shortcut.
Cursor was in search field, the search field was empty.
A 'xxxx-Tab' created/inserted a tab into the search field; when pressed a second time, the focus was set to the 'Create an orphan thought'-button; when pressed a third time, focus and cursor was set back to the search field, right behind the tab created before ...
Btw: when 'Alt-Tab' is pressed in notes, the cursor disappears - but text can still be entered ...