The space between things
Example article. Sample writing for this page.
Space gives related ideas a place to sit together. A heading closer to its paragraph feels connected to it. A larger gap before the next section tells the reader that something has changed.
This becomes easier to notice on a small screen. There is less room, but that does not mean every gap should shrink. The space around a button still helps a thumb find it. The space between paragraphs still gives the reader a moment to pause.
Before adding another line or box, try changing the spacing. Move the heading closer to its paragraph. Give the next section a little more room. Then look at the page again without reading the words. Its structure should still make sense. Sometimes that is all a page needs.