If there is no Special:Preferences redesign task, there should be. Beta features look out of place at current page and OOUI already has all elements to redesign the page.
This is probably OK, though, the less (gradients) the better.
Screenshots from above patch. This is only caring about the fallback color when no gradients are supported.
|without any gradient or image without any gradient, but fallback image||without fallback color|
Removing the blue gradients provides a cleaner look that is more aligned with our visual style principles.
I don't think there should be a problem recognising the elements as tabs. The only drawback I see is that the contrast between the selected element and the unselected ones is reduced with the change: it may be easier to distinguish between white and blue than distinguishing between white and light grey. This is an aspect usually emphasised in tab usability guidelines:
Highlight the currently selected tab. Make sure that the highlighting is prominent enough so people can tell which tab is selected.
Given that the text color is different for selected and unselected tabs in our case, that may help to increase the contrast, but we may want to pay attention to possible issues in that regard.
I dunno, the tabs really do need something to distinguish them from both the active tabs and the actual page background, or they all just kinda blend into whichever they do match...
I think maybe if you want to fit the style guide, just not using Vector might be a more reasonable approach. The skin as a whole really wasn't made for this kind of design to begin with.