Hi Pilou, the grid color and also the viewport background color are preferences you can set in the Options dialog, not something that is defined by the UI files. So yes that's normal that updating the moi.css file will not affect grid color.

The grid color can be set under Options > Grid > "Grid lines" and "Accented grid lines". Click on the color swatch next to the "Color:" label to set those.

The viewport background color is set under Options > View > Colors > "Background color".

- Michael