CSS overrides other extensions - partial fix

See darrinhenein#13



SCENARIO:
Firefox and tab Mix Plus. Set up some nice garish background colours in T.M.P. for loaded/unloaded/unread tabs. They work fine. Now install VerticalTabs. They immediately lose their T.M.P. colours.

CAUSE OF ISSUE:
There's a css issue in the theming css files (win7.css etc). The extension doesn't just handle vertical tabs, it also expects to recolour tab borders/backgrounds to "default colours" or "VerticalTab theme colours" (Why, why?!) when recolouring is completely unrelated to the extensions job of vertical formatting. This kills any tab background colour or formatting by extensions like TabMix Plus, ColorfulTabs, etc.

SOLUTIONS/WORKAROUNDS:

Solution 1 - just kill all theming to be honest. Vertical-ing a tab bar doesn't mean re-colouring it.

Solution 2 - remove all "background" color code from the Win7 and other OS css files. For Windows this means removing (at least) these lines:

.tabbrowser-tab {
-moz-box-pack: start;
-moz-box-flex: 0;
-moz-appearance: none ! important; <------------- DELETE THIS LINE
background: transparent ! important; <------------- DELETE THIS LINE

Those two lines seem to fix it enough for ColorfulTabs and others to semi-work. Really needs to kill all theming though. 
Vertical tabs should do just that - not enforce colours as well.  The colours should already exist in the current tab bar (or be copied from it by the extension), or some of the !important's shouldn't be there.

skin/win7/win7.css

@@ -29,8 +29,6 @@
 .tabbrowser-tab {
     -moz-box-pack: start;
     -moz-box-flex: 0;
-    -moz-appearance: none ! important;
-    background: transparent ! important;
 
     border: 1px solid transparent;
     border-radius: 2.67px 0 0 2.67px;