When I log into my Confluence, the CSS from /styles/main-action.css looks like this:

----------------
body, p, td, table, tr, .bodytext, .stepfield {
font-family: Verdana, arial, sans-serif;
font-size: 11px;
line-height: 16px;
color: #000000;
font-weight: normal;
}
#PageContent {
text-align: left;
background-color: #fff;
padding: 0px;
margin: 0px;
padding-bottom:20px;
}
/*

• • when this stylesheet is used for the FCK Wysiwyg editor's edit area, we can't
  • use an id=PageContent or class=wiki-content, so we must
  • set the body style to that used for PageContent, and p to that used for wiki-content.
    */

body {
margin: 0px;
padding: 0px;
text-align: center;
background-color: #f0f0f0;
}
------------

Notice "body" is defined twice. Now, if I to any change to the space labels (add or delete a label) the CSS changes to:

------------
body, p, td, table, tr, .bodytext, .stepfield {
font-family: Verdana, arial, sans-serif;
font-size: 11px;
line-height: 16px;
color: #000000;
font-weight: normal;
}

body {
margin: 0px;
padding: 0px;
text-align: center;
background-color: #f0f0f0;
}
-----------

So the "#PageContent " and the comment goes missing. This causes the background of the installation to start using the f0f0f0 (grey) background color so the whole confluence looks odd. The color reset happens to both Safari and Firefox, both when I'm logged in and/or browsing anonymously. Strangely if I go to the global admin section and reset the color settings of Confluence to the defaults (which they already are), the background resets to white again.