I found the solution. I made a flow using the card “Dim Hold Up”. In the than column I added the philips hue light “relative brightness”. In this case I used +21%. So it will get brighter in steps of 21%. I added a similar flow to lower the brightness.
With the percentage you can adjust the speed (step-size) of the dimming. In the screenshot I use 2 bulbs, as there are 2 bulbs in the same room. I control them with a single remote.
Total I have 4 flows for this remote:
-On button --> turn on
-Off button --> turn off
-DIm-up button --> Dim-up using relative brightness
-Dim-down button --> Din-down using relative brightness.
Like this all buttons are working as they are supposed to 
