Well, as I said both works will be used.
I'll implement icon theme support.. it's fairly easy if no UI is involved.
You just have to change a line in the config saying something like:

theme="default"

and change default with the theme name.
I'll expect icons in the directory:

XOURNAL_DATA_DIR/Themes

(/usr/share/xournal/Themes)

with a name for each theme:

/usr/share/xournal/Themes/default/

and then the icon names:

pen.png
eraser.png
highliter.png
ruler.png
hand.png
and so on...

Shouldn't tale long to implement