I'm coding my own custom install.php (wp-content) and I want to be ablt to activate my custom theme when an install is running. Right now I've tried switch_theme('theme-slug') which works, however when I first log in to my admin panel, all the admin pages under Settings etc. is gone, so something is not right. If I remove switch_theme function from my install.php, then it tries to activate "twentyseventeen" however it can't find that theme, and that's because I've removed it from the themes folder, because I want to activate my own theme called "mastertheme".
Can somebody help me on how to activate my own theme when WordPress installs?