So bulletins are a pain in the ass. The way the site works, when you post a bulletin it adds a <br> tag to the end of any line. If you want to do any css in a bulletin, you have to do all of it on one line
You can accomplish a background with something like this:
<style>body{background-image: url('your-url-here');}</style>
(I haven't tested this specific css but I think that's right)
Obviously putting everything in one line can cause problems, so I recommend using a site like neocities to create a .css file and link it to your bulletins using an import statement:
<style>@import url('your-url-here');</style>
This way there's less of a chance spacehey's parser will mess things up. Whatever you put in the file available at the url will be imported and applied to the page