Has anyone done this at night, and if so, how hard is it to see the road signs withour getting lost? We are older and not very good at traveling after dark.
I haven't made the drive after dark. However, the drive is relatively simple as you'll need two roads: E65 and 1. Take E65 out of Zagreb. At Karlovac, exit and take 1 south. That road will eventually lead straight into Plitvice.
The A1/E65 autocesta to Karlovac is a four-lane divided highway, akin to an autobahn or interstate in the US. The D1 (or simply 1) national road from Karlovac onwards is a regular two-lane highway. There is some traffic as locals use it to avoid tolls on the A1 to Split, but it is a perfectly adequate road in my experience. However, if you are uneasy about driving in the dark, consider spending the night in Zagreb and leaving for Plitvice earlier in the morning instead. Driving in a foreign country with the different lane markings, signposts, etc. just adds to the disorientation, even if the roads may not be worse per se.
You will be super fine. E65, E71, D52 is the safest way to plitvice lakes. All regional main roads. Only winter time can be hard.
Have a safe trip!