Hello,
Thansk for using XStore.
It is recommended to run import on fresh WordPress installation. For this you can use WordPress Database Reset plugin https://wordpress.org/plugins/wordpress-database-reset/ The plugin can be used to remove all the existing content and install a new version from scratch. It is recommended to disable the theme https://prnt.sc/unle03 before the reset.
Sorry, but I am not sure that fully understand your request about a few themes. By default, when a theme is activated, all pages of sites use this theme.
Regards