Something simple but still quite effective is to change the colour of the login window border.

Look for the following five .png images in /templates/base/images/

sp_op_gr.png
op_cor_tr.png
op_cor_tl.png
op_cor_br.png
op_cor_bl.png

Using something like photoshop, change the hue/saturation to whatever colour you like best. When doing so, if you save the colour you chose by clicking save in the hue/saturation popup then if you want to reference the exact colour in the future, all you need see more do is click on load and choose the file you saved previously.

It's up to you whether you keep the images as opaque/transparent or as solid images.

One more hint to save you time, make a copy of the image folder just in case you stuff up and need to start again.