Toolbar Style Guidelines for Office Add-ins

[mrb]

I'm currently building an Office Add-in which adds a commandbarbutton and
drop-down combo box to the 'Standard' CommandBar in: Word, Excel, Powerpoint,
Visio and Project.

I would like to customise my own CommandbarButton Icon and would like to
know whether there are any Style guidelines for customising Tool bars in
Office Applications specifically (e.g. Image Size, Colours etc.).

Any documentation / pointers would be be helpful.

Thanks,

Andrew Bewick