I think Sheryl means that the preview image is not showing. See https://wordpress.stackexchange.com/questions/27427/wordpress-theme-preview-image. Create the screenshot yourself and place it in the theme/child theme folder and name it screenshot.png
I think there maybe a lot of reason but first can you please check is there any updates available in your WordPress and then please check? You need to upgrade it to 5.6 or newer then 5.6 and then it will work.
I’m not sure I fully understand as the question could hit a few different scenarios. I’m assuming for localhost you mean you’ve developed the application locally, therefore you could move your local application to the live site – for a simple guide on how to do this check out: https://www.wpbeginner.com/wp-tutorials/how-to-move-wordpress-from-local-server-to-live-site/ The idea of local development … Read more
There should be an option to add custom CSS to the theme that will over ride the styles. Normally found in your theme settings. If there is not a custom CSS area, you can access the CSS somewhere around Appearance > Editor. If you then navigate to style.css you can apply changes there. However using … Read more