https://git.wikimedia.org/blob/wikimedia%2Fbugzilla%2Fmodifications.git/HEAD/bugzilla-4.2%2Fskins%2Fcontrib%2FWikimedia%2Findex.css

Something in or under .bz_common_actions probably needs the line

margin: 0px auto;

It looks like changing the CSS for .bz_common_actions by
adding

margin-left: auto;
margin-right: auto;
max-width: 830px;

and removing

float: left;
margin: 0px auto;

might work.

Testing very welcome.

Change 82247 had a related patch set uploaded by Aklapper:
Center the 'Common actions' buttons on Bugzilla frontpage

https://gerrit.wikimedia.org/r/82247

Tested on http://boogs.wmflabs.org/ with Firefox 23.0.1, Google Chrome 29.0, Opera 12.16 and different zoom levels. Seems to work.

Testing with an old Internet Explorer on http://boogs.wmflabs.org/ highly welcome.

I've tested this change on boogs.wmflabs.org with IE on crossbrowsertesting:
IE7 on Windows XP SP2 800x600 and 1400x1050 work (no CSS issues, all text readable, links can be clicked).
IE6 on Windows XP SP2 1400x1050 looks like crap (no news) but that's no news (see bug 28871) and the page is functional (can click links).

Change 82247 merged by Dzahn:
Center the 'Common actions' buttons on Bugzilla frontpage

https://gerrit.wikimedia.org/r/82247

Thanks!