No, it looks good in the default style
Please note that the sidebar width in itself is not the issue, as I have changed it from the default 270px to an exaggerated 350px - to no avail.
Also note that my theme disables the icons in the block header but I doubt that that is the issue.
The issue MIGHT be, as I said, that the 'shadowing' of the inner 'latest contributions' header being larger than the container it is in ... but I'm no developer so I'd rather not mess around unless instructed to by the professionals