Abo-abo has made a slight enhancement to ace-window that makes it possible to customize the face of the selection character of each window. Take a look at the example in abo-abo’s post to see what I mean. It makes the character much easier to see and looks very nice.
If you’re trying to squeeze the last drop of efficiency from your key bindings, notice how abo-abo has mapped the selection characters to be a s d f g h j k l rather than the default 1 2 3 4 5 6 7 8 9. That means the selection keys are on the home row and very easy to reach. Another micro-optimization that helps make Emacs use as frictionless as possible.