Hi,
I have read the other questions and answers about this however I still couldn't make it work.
The default row height is 36px. I want to make it smaller. I use RowRender action on the DataGrid. If I make it greater than 36px, it works fine. But when I set the row height smaller than 36px, nothing changes.
Even playing with dev tools doesn't affect anything. Any idea about what I'm missing?
Padding and line-height ...
By default there is no height set. The td has 0.5rem (8px) padding. This combined with the default line-height of 20px makes for the 36px height. So to decrease it you have to set your own padding (less than 8px) and line-height.
From inner to outer:
I had already set padding to 0. Even if I set a line-height on the tr or td tag, it doesn't change anything.
Because rz-cell-data has line-height: 1.5 (visible in your first screenshot).
I know but it has no effect on the row height unfortunately. It only changes the span height.
td stays the same
This is now what I am seeing.
You have the proof that it works indeed 
Somehow, when I try the same thing on my project, nothing changes. I will remove all custom css classes and retry.
Thanks korchev.
