Hi Shlomi,
as far as I know, after loading a CSS file to Launch pad, you cannot edit that file directly there (can't open it in edit mode directly from launch pad and can't open the file in edit mode in Design Studio either).
You have to indeed make all changes locally and upload the file each and every time you want to see the CSS changes in a Design Studio report via Launch pad.
What I usually did so far (but I am not developing reports just testing new features of Design Studio from time to time): I do the development always locally and only upload the final product to the BI Platform via "Tools > Upload Local Application...".
But it would be a helpful feature if it would be possible to open a CSS file from the BI Platform in BI Platform Mode directly in Design Studio and when all changes would be directly saved to the BI Platform. You could add this as an idea here so that others may vote for it and SAP considers it to add it in a future release.
Regards
David