Compile less to css

Hello,

I'm new here.
I'm using the t3 blank template
Now I have turned the Develop Mode on and saved it.
I made changes in the css files (templates/t3_bs3_blank/less
I turned Development Mode off and saved.
Then I compile less to css

While in Development Mode I see all my changes.
With Development Mode turned off all my changes are gone.

What do I do wrong?

Thank you already and sorry for such a beginners question
(I have seen the videos and read the documentation but keep getting this problem)