is there a way to make my profile background less terrible to look at but still what it is

So if you go to my profile your eyeballs will be violently assaulted by rainbows... I want to keep the rainbows because I am a rainbow, but I also don't want to cause an increase in eye damage in you lovely peoples

is there a way to like.... add black behind it and decrease the transparency of this image a bit? I am an artist of non-animated art only, so I understand what I theoretically want to do here, but I have no idea how to put it in words and certainly no idea how to put it in code... please give me advice on how to make this less awful, because I do love this background so much!

maybe thick black borders on the edges of my profile would help so it's not quite as distracting from the main info? but then how do I do that?!?

sorry in advance to everyone who views my page right now... it's a work in progress starting from a mismatch of the most rainbow layouts i could find... I am a Myspace refugee so it'll be a gorgeous scene mess eventually i swear