custom CSS fails to save at all

Ok here's the low-down:
My forum's theme uses a gradient image for a background, and it tiles vertically. This is ugly on large screens, so I decided to replace it with a CSS gradient. I know the code is correct, because I've used the generator I used on other projects.
In this case, I opted to just copy-paste the whole basic CSS from the theme, and replace the background image bits with the gradient:

Spoilered for size:

Now, I made sure that "Deactivate basic CSS" was set to yes, and saved the stylesheet.
Clicking "Submit" sent me to a completely blank page, so I reloaded. Stylesheet showed up completely blank. Just to check, I reloaded the forum itself. No styling.

What might be causing this, and is it something I can fix?

All you have to do is add just the part you want to change.  The stylesheet overrides the basic CSS.

thanks, that seems to have worked.