Once you have Redcar installed and running, you'll be presented with a rather empty looking window. That's OK, nothing is broken. Go to File->New or File->Open to get started.
First off, let's change the theme. Redcar comes with many themes, one of them is bound to suit you. Go to View->Appearance->Theme to open up the theme selection dialog. You can look through, click on ones you like, or click on the top one and use the up and down arrow keys to try each one out.
Next, the font and font size. Go to View->Appearance->Font. This dialog is similar to the theme selection dialog. You'll notice that you can actually use fonts other than mono-space fonts. Many editors don't let you do this, or are quite glitchy when you do this. I've had no problems using fonts other than mono-space fonts. Finally, go to View->Appearance->Font Size. This is a simple dialog with a single text field, nothing special but it does the job.