I haven't given up yet. 
It would take less tweaking of the theme if you just had larger versions of the current PNG images that are used for the buttons. It wouldn't be as nice as having vector graphics, but it would take less tweaking of the theme.
I used Imagemagick with adaptive-resize, which is supposed to reduce the blurriness of scaling up images, to scale the close.png image up to 39 x 39 pixels. (Its original size is 19 x 19.)
To my eyes, it doesn't look that blurry, and 39 x 39 is probably larger--and more blurry--than you will need. With larger images, you could do the hack described here, and perhaps have buttons to your liking.
I wrote the following bash script to make it easy to resize all the images in a folder, but before using it, note that it uses "mogrify" instead of "convert," so it's scaling the images and saving them to the same file names. You might want to create a backup before you run it:
#!/bin/sh
# How many pixels do you want to scale the PNGs?
echo "How many pixels should each PNG be scaled up?"
read scale
# Scale the PNGs.
for fname in `pwd`/*.png; do
# Get the current dimensions of the image.
width=$(convert $fname -ping -format "%w" info:)
height=$(convert $fname -ping -format "%h" info:)
newwidth=$(($width + $scale))
newheight=$(($height + $scale))
mogrify -adaptive-resize "${newwidth}x${newheight}" $fname
done