Open MenuSingle.szs in Brawlbox. If you don't have this file, you can find it at https://avsys.xyz/MKW/MenuSingle.szs

After opening in Brawlbox go to button -> timg -> tt_mario_64x64.tpl and right click Texture0 and hit replace.
Replace the image with CSS Icon in this folder, and then save.

Do these steps again but now with MenuMulti.szs. You also need to do this in Race.szs, but instead you need to go to:
game_image -> timg -> tt_mario_64x64.tpl. To change the map icon, in the same directory, go to st_Mario_32x32.tpl
and replace with the Minimap file in this folder. 