The style tag isn't allowed in bulletins, but you can add something called inline CSS. That just means you can add CSS to regular HTML tags, like divs or imgs.
Here's code for adding a transgender flag to the background. Just add this anywhere in your bulletin post.
<div style="position: fixed; left: 0; top: 0; z-index: -100; background: skyblue; width: 100%; height: 100%;"></div>
<div style="position: fixed; left: calc((100% - 810px) / 4); top: 0; z-index: -90; background: pink; width: calc((100% - 810px) / 2 + 810px); height: 100%;"></div>
Feel free to modify or do whatever you want with it! Just be careful with the z-index property, it should be negative so that it stays in the background and doesn't block important SpaceHey stuff.