Figured I'd write about this as it's been the single thing holding me back from writing a blog in Squarespace. This method leverages Alex Gorbatchev's hosted SyntaxHighlighter. Hosted as in referencing the .js and .css files from somewhere else, info on that here. The below screen shot shows my Squarespace code injection config. This setup is pretty simple, but from googling around there are quite a few <style> modifications you can make. You can see below that I've specified the theme RDark (other themes available here). I've also pulled in the PowerShell brush (other brushes available here). You need to take note of the brush alias for later, PowerShells is "powershell" or "ps".
This is how it renders, wow.
Credit | References
http://iphonedev.tv/ (Paul Solt)