Sorry, filemods are not entirely identical with the normal slideshow features. The filemod slideshow inherits the frame is enabled, but it does not use frame border.
You can use the native slideshow for your start page if you want, and then it will inherit the same styles as the other slideshows. To do this, first you need to set your start page as "gallery" (instead of filemod). Then under "parameters" for this folder, add the following parameters:
settings.startmode=image&image.slideshow.autostart=true&thumbnails.thumbnailscroller.enabled=false&controls.largecontrols.enabled=false&controls.maincontrols.enabled=false&image.imagebuttons.enabled=false&image.closebutton.enabled=false
You may want to modify the parameters above, but this is the basic settings to make it look like the slideshow filemod, except it will inherit the same style as your gallery slideshows.
More about folder parameters here:
https://www.photo.gallery/documentation/folder-parameters/
