Over-riding a templates background image

Hello,

I'm just wondering if you can help. I am a first time Squarespace user but quite competent with HTML and CSS.

I'm using Squarespace 6 and the Template Aviator, its a beautiful template so well done Squaresapce!

However I am keen to over-ride the background on the home page and its my understanding that I need to inject the CSS into the header as I am using a template and not a developer site.

As an example I've put this into the heder in the code injection settings:

<style TYPE="text/css">
      body
{
background-image: url(https://someurl.com/someimage.jpg);
background-color:#cccccc;
}
</style>

However this isn't working, well it is but the original template CSS is over riding it. I can see my background image for about 2 seconds then the original BG loads.

Any advice for this newbie?

Thanks!

Ben