Not easy to see at that size but...
if you click on it, it'll be better to see
Is there a ground plane? Doesn't look like it because it seems to have tracks connecting the ground pins I can identify.
You were right, somehow I thought it was right. Found also a good trick to see if the gnd traces go into the groundplane by using: run length (it shows zero length for gnd, so they go all into the groundplanes I assume)
hat is the fuse on the left supposed to do? Does this have high current through it or is it just protecting the Arduino?
it's adviced in this thread to use two fuses. And this fuse on the left is getting 8v from the 7808 into the raw arduino input.
I wold suggest putting the fuses together on the right side - make a kind of firewall between the unprotected inputs and the rest of the circuit.
Don't understand what you mean by firewall between the unprotected inputs. 12v in - fuse - 7808 - fuse - arduino raw in.
You have fat high-current tracks going to one pin on each output connector. What about the return current or does that go off-board to a remote grounding point?
12v to feed ledstrips on each output connector. Using irf540 to pwm ledstrip on gnd output pins.
It looks like you used the auto-router. Can you see what it did there? All the top tracks are 90 degrees to the bottom tracks. If you are laying it out manually, try to follow this 90 degree idea - make all the blue tracks vertical and the red tracks horizontal.
Wouldn't this create alot of VIA's ?