In WP navigate to Appearance > Themes > disable your theme by activating another one (default theme preferably)
Next > via FTP rename your theme folder to something unique > reactive your theme in WP appearance > themes
In the source code you should now see your custom theme name. Please note, some themes have hard-coded paths which may break internal linking for css/font files. I recommend that you make a backup of your entire theme folder locally before you try making these changes.
There is no way to hide your theme from the source code as it will have to include your theme name to load its files. Why are you trying to hide this information?
Next > via FTP rename your theme folder to something unique > reactive your theme in WP appearance > themes
In the source code you should now see your custom theme name. Please note, some themes have hard-coded paths which may break internal linking for css/font files. I recommend that you make a backup of your entire theme folder locally before you try making these changes.