Ctrl + 0, Ctrl + Plus Sign and Ctrl + Minus Sign are the designated keyboard shortcuts for controlling the zoom. This works on my setup.
Ctrl + Shift + Minus Sign seems to work identically to Ctrl + Minus Sign, but on my setup, Ctrl + Shift + Plus Sign does nothing, whereas Ctrl + Shift + 0 does what, given Minus Sign’s behaviour, I’d expect Ctrl + Shift + Plus Sign to do: it zooms in.
I’m guessing this is keyboard layout dependent. I’m using the Finnish/Swedish keyboard layout, in which the ’0’ key with Shift is the = character. In US keyboard it seems the Plus Sign has its own key, and that key with the Shift pressed becomes =. This is why Chromium thinks that I’m doing the ’zoom in’ combination when in fact I’m doing the ’reset zoom’ combination with shift.
I came across this while looking for a way to reset the zoom back to my custom default via keyboard. I figured if Ctrl+0 is ’reset to normal’, Ctrl+Shift+0 could be ’reset to default’. (I have yet to find such a keyboard shortcut. That’s a separate issue from the one I’m filing here. I’ll have to file a wishlist bug if said shortcut doesn’t exist.)