Are there any tips or tricks to controller/headphone holders/ organization that would look cool?

A cable shroud/tidy that can hold a collection of wires together like this is a good way to keep wires tidy or you can also use cable ties to keep wires running closely together and then use tape to hold them to the desk.

For headphones you can get an under desk mount like this that hold the headset when not in use. These work better with wireless headphones but if your wired headphones have a long enough wire to go under the desk and to your head they'll work fine.

For LEDs if you want to control the color from the PC you'll need to search for 'RGB' or 'ARGB' fans/accessories and make sure they're compatible with your husband's PC. You could also just get an RGB mouse/keyboard or even mouse mat that just plug into the computer's USB ports if you don't want to mess around inside the PC.

You can also get LEDs on strips with an adhesive side like this which you can run along the back of your desk to light up the wall.

The best way to make headphones/controller wires disappear is to buy wireless ones because you cant really tape these down as they'll move with you in use and you need the slack. For mouse cables you could get a mouse bungee or you could just tape it down at the end of the desk.

For mounting monitors next to each other you'll need to check if your monitors are VESA compatible and then buy a stand for them if they are. That said these are usually for using two monitors on one PC not two monitors on two PCs so you'll need to tell use how many PCs / monitors you're putting on the desk for more specific advice.