Yes, I found the Backlight Intensity dial eventually under the Display button, H,M,L.
As the LED strips are 3 times longer as needed, and I had another TDS3012, I decided to give that a go also. My learnings today: best to get rid of the chopper board and just power the LED strips from DC with a small current limiter/controlling resistor. I picked the +5 and -15V rails to get 20 VDC, then the two strips in series and 150 ohms as current control, 25-30 mA is good enough for me already, but maybe go much higher if you want more light. The strips have every 3 white LEDs in series, so 3xV_LED, and that's 7 to 9 V ish)).
I think it's better anyway to not have a LED chopper circuit inside the scope, I have the impression that in the first one I did, with the PWM chopper, I now see more noise/ripple on my channels as before...
Plus: do re-use the metal frame that had the old CCFL glass tubes, then no need for double sided tape for holding the strips in position anymore
Plus: leave out the entire high voltage chopper board. The scope works fine (for me) without it.