It is easy to change video player appearance with lightgallery. For youtube videos you can refer the official youtube documentation here. For vimeo videos please refer the official vimeo documentation here.
You can automatically load thumbnails for youtube/vimeo videos with the help of lightgallery thumbnail plugin. You can easily specify the size of the thumbnail by passing thumbnail size via youtubeThumbSize
/vimeoThumbSize
settings. Here is the full list of available thumbnails.