What type of custom backgrounds are you looking for? Are you just looking for the gradient ones that everyone uses, gif themes, personalized image bg's, or just the 1 color backgrounds? I don't play other servers so I'm not sure what type of custom backgrounds they're using but on Classic, a lot of players use gradient backgrounds.
For gradient backgrounds, you need an art program like Art Studio, Photoshop, Gimp, Paint.net and etc.. Once you open your preferred art program, it'll ask you what dimensions you want your file in(pixels,inches,centimeters,picas..). Since you're going to be using that file for Graal, just select the one that says
pixels but you also have to make sure one of the width/height is set to 1 pixel while you're at it. So for a
vertical gradient background, you would have to set your file to 1(height) x 4(width) pixels. And if you want a
horizontal gradient background, you would just set it to 4(height) x 1(width) instead. The 4 is just an example.. you can choose whatever number you want as long as one of the width/height is set to 1 pixel but that's basically it. And then you can just upload the file through your phone or pc when you're done with it.
Here's just a little example.. It's not actual size because i enlarged it but the original file's height is 4 pixels, by 1 pixels width.
Once it gets uploaded on graal, the file gets stretched out and blurred(don't have a guild to upload the background on so I just gave an example instead of screenshot, which is shown in the 2nd image).
If this isn't the custom background status you're looking for, feel free to post an example of someone elses' profile from the other servers that you play, and we'll help you figure it out.