On many sites, I can't see the slight color change that indicates I've already voted unless I look very carefully. This seems like a massive accessibility issue, given that my vision is above average and I'm still struggling to use the UI.
The color change between the up and down arrows is almost indistinguishable to me, especially at the now very small size of the arrows on my monitor. The background seems to be slightly darker, and the border is darker, but these are details that are much much more subtle of changes than the old UI.
The specific colors chosen seem like the main issue. The background is just 3.5% darker when the button has been clicked, the saturation of the color used for the arrow is less than 50%, and the lightness of 41% is rather similar to the 25% of the unclicked arrow. While if I zoom in several times the difference is clearer, I don't always have the luxury of seeing my screen right up close.
This could be easily remedied by using a brighter, more saturated color, and bigger difference in brightness. For example, here's a (very rough) mockup where I've just increased the saturation and lightness of the theme color, and the saturation of the background:
Upon further inspection it seems the color used for the buttons is the site's theme color; this worked fine when the buttons were huge solidly colored blocks, but seriously needs to be looked at again when the only solidly colored part of the button is smaller than the number below it, especially for sites like those using the default theme where the primary color is so close to dark gray.