Hi Alex,
Here is a patch which allows zooming in and out using the Refresh button. Right clicking on it zooms the page in. Middle clicking zooms out. It's not especially intuitive, but it's simple and it works.
I agree is not very intuitive, maybe using the scroll wheel may be more clear. But still, tying it to the refresh button is weird to me. On the other hand, I like that you can see the whole page while changing the zoom level using the mouse (which doesn't happen in other browsers). I'm thinking if it could make sense to add another "appearance" bar (usually hidden) like the "find text" where we can put some knobs to change the page appearance, like the zoom buttons or sliders but also maybe some of the CSS options too. I think this would also make more sense if we can select multiple "user" styles to implement some king of reader mode, as you will want to see the effect without covering the canvas. Rodrigo.