I haven't had experience with Arty's style but looking up in firebug the node icons are using the spritesheet styles/soft_responsive_coffee/xenforo/node-sprite.png
You'll most likely need to either add your own spritesheet and overwrite the arty's spritesheet or of you are using individual icons you'll have to change the path and set those like we did before on your other site directing the path of the images to the ones you are wanting to use.
Another note, if the custom icons differ in size you'll have to alter the height and width dimensions from within the Style Properties >> Node Icons adjusting them accordingly.
I haven't had experience with Arty's style but looking up in firebug the node icons are using the spritesheet styles/soft_responsive_coffee/xenforo/node-sprite.png
You'll most likely need to either add your own spritesheet and overwrite the arty's spritesheet or of you are using individual icons you'll have to change the path and set those like we did before on your other site directing the path of the images to the ones you are wanting to use.
Did you make them into a spritesheet? or using them individually? If spritesheet rename it the same as arty's spritesheet and overwrite ensuring you've replaced all icons including the page and link icons. If your using individual icons your going to have to change the paths and direct them to those icons in Style Properties >> Node Icons >> Node Icon: forumNode Icon: Unread ForumNode Icon: PageNode Icon: Link
I've included two of my own designs (page & Link) node Icon but simply go to this directory on your server >>styles/soft_responsive_coffee/xenforo/node-sprite.png
And upload the spritesheet in the attachment and overwrite.
I was out in the garden enjoying the great weather and doing more gardening so very refreshing until I got to the strawberry plant I planted 3 years ago that has never gave up a strawberry, that was a little traumatic.