How to change indent size for indent guide line

after I enabled indent guide line, the virtual vertical line will display in the ATOM editor. It seems the indent size is forced to be 4 spaces, however my preferred size is 3 spaces.
I googled, and all i got were how to set soft TAB size. Can anyone tell me how to change default indent size for indent guide line?

There is a global setting and each grammar has an override setting.
The config file contains these settings if there are doubt.

The global default can be seen:

Some tips on where to see the grammar setting; one example: