It is hard to read the amount of the bid when the item is highlighted/selected with the dark blue back-colour.

NEW, SIMPLE ANSWER:
The selection color is inherited from your operating system (i.e. Windows, Mac OS X, Linux, etc.), but it can be configured. In the Window menu bar item, there's a menu item 'Change Selection Color' with which you can configure the background color for selected lines.


OLD, COMPLEX ANSWER:
The selection color is inherited from your operating system (i.e. Windows, Mac OS X, Linux, etc.), but it can be configured. If you go into JBidwatcher's Configuration Manager (the two checkbox icon, or File | Configure), and go to Advanced.

For the key, type in selection.color, and for the value, enter a hex representation of the color you want, in RRGGBB format. This might not mean anything to you, so here are two colors that might be good to start with:

C6A646 for a slightly brown/tan-ish selection (my default on Windows) and
999999 for a medium grey selection.

Some outrageous ones are:
cc0000 – bright red
00cc00 – bright green
0000cc – deep blue (probably even more unreadable)

You could also use a web color picker and take the values out of the background box under the palette.