FrameMarket: creating new site in multisite getting server error

We have a multisite but our problem is when we create a new network and visit the newly created site, it gives us an http 500 error. We found out that this is caused by the stylesheet reference because our theme is a childtheme. If we go to the settings of the network and change the stylesheet of the theme to its theme then the error disappears. What we want to do now is to make the stylesheet reference fix in code, but the problem is we don't how to edit this in code. Is this possible?is there any settings for this? Is there anyway we can make this happen? Please I need your help.