Thank you for your post!
In general, moving from the Classic Theme is a good idea. It has been deprecated and removed in the latest versions of FileMaker. You could potentially run into issues going forward in Classic Theme.
You might try hitting CMD + Z for Mac or CTRL + Z on Windows after changing the theme. This will try to size things back to your previous design. You may still need to do some resizing, positioning and field formatting, but it helps a lot.
Well, first off, I wouldn't use Classic. It's deprecated. I'd use Minimalist if you want a basic theme with no styling.
But beyond that, some of the formatting associated with various layout parts is embedded in the theme and can't be changed. So you can either use another theme (like Minimalist) or just use a line object.
Dear TSPigeon and Mike,
Thank you for your answers. Duplicating the layout and choosing a new theme and then copying all objects of the 'old' theme works in my case. As I have made all layouts of all databases up to now in this deprecated Classic Theme, this is although some work.
I hope that the theme I've chosen ('Minimalistic') will last some time...