Yes, those things can be changed. Each gallery has a set of css files that control how it looks. Each one has a common.css which controls the overall layout. Then each has a dark.css and light.css. I would recommend making a copy of the dark.css and name it something like custom.css. After creating this new css file, you should see an additional custom option on the Choose Design page. Just select that and use the code from Add to Site. Go ahead and change the colors to anything you want in this new css file.
In terms of changing the thumbnail size, that might be a little more tricky. You would have to update some of css for you gallery and also adjust the image thumbnail size (either in the css or html in index.php for the gallery). It can definitely be done, you'll just have to adjust some of the sizing until you get the effect you want.