div having different appearances in different themes

You need to be specific when defining your CSS styles so that a theme will not overwrite your intended style. When outputting your markup, perhaps you should even consider wrapping your div element within another div element and assuming this is the only markup on the page, you can specify them both with an #ID … Read more

spacing disappears on posts when importing

As Sven noted in the comment, your theme doesn’t have any spacing after paragraphs. It’s actually set to no space between paragraphs on line 797 of your style.css. If you were to change that line and the one above it so the last numbers in each line were 10px and 1rem, you’d have fairly default … Read more