The ctl (5) pin on the 555 is a handy method to alter duty cycle (per link in post #2). Send the PV output voltage to this pin. No doubt it's more complicated than that because it's likely you'll need to apply gain, level-shift, etc.
To attenuate use a resistor network, or zener, etc. A 12V rated panel might rise to 20V or more with light load. It probably is bad for the 555 IC if you expose any pin to a voltage greater than the supply voltage.
Amplify voltage (or level-shift) through an op amp. Etc.
It will help if you experiment by applying various volt levels to the 555 ctl pin. See how it affects duty cycle. More tests should produce the desired brightness of the led's, for daytime and night. The aim is to tap the PV voltage, and use it to alter brightness.