Feature : Better per client colors for graphing activity

[0xc0ffea]

Small network, pi hole , client activity graphs on the dashboard are fairly useless.

For some reason, the PiHole has decided assign all but one of the clients pale blue, which makes them indistinguishable at a glance.

I am not requesting the ability \ ui to manually set a client color, that's overkill.

[dschaper]

#1250