Created attachment 109256 [details]
The formatting toolbar has 3 or 4 cell number formats in it, though there are 10 number formats in the format cell dialog. So in order to make these easily available in the toolbar, as they are in the sidebar, a simple drop down control should be added to the toolbar.
I like the idea of a drop down, but would prefer an icon + list..
See e.g. Paste-special.
Why i was suggesting this way over the icon style is that similar to the font name and font size, you can type the name of the format to easily select it, and when you select a cell with a formatting, the name of that formatting will be easily shown in the field's area.