Nods at Flint. The C in CSS is for 'cascading' after all.
Meaning if you have a
Code:
.infoBox h1 {
font-size: 1rem;
}
in the main theme, and you want this to be bigger in the child theme, simply "overwrite" it by adding
Code:
.infoBox h1 {
font-size: 2rem;
}
in the child theme. The CSS loaded at last will take precedence, and the very final CSS to be loaded is whatever you put directly into the markup, like style="font-size: 2rem;" (but you really don't want to do this unless absolutely necessary.)
This also goes back to what I said about mobile. If you look at our current CSS, there are a section of @media-tags at the end of the CSS sheet, which refers to width of the screen. For wherever you want stuff to look different in mobile, you can use that kind of logic to make certain elements stack differently. So when I say "keep mobile in mind", it's more - keep in mind that if you make changes to how stuff stack on desktop, or change sizes or widths or whatever, remember they probably need to be adapted to mobile as well.
Have you ever used the "Inspect Element" addon in Chrome? I highly recommend it. Press F12, press the mouse icon and select whatever you want to inspect (or right click on an element on the page and click "Inspect Element"). CSS will be listed on your right side, referring to which file it is loaded from. The CSS on the top is the last to be loaded. If you untick the boxes, you will be able to see what the page would look like if you removed part of the CSS. Very handy.
I use that tool on a daily basis, not just for development, but also in meetings to give a quick, visual demo of what something would look like, or to point out problematic things about what they want. Outside meetings too; I change stuff in the add-on, screenshot it and put it into our issue system. That way I don't have to touch GIMP/Photoshop at all. <-- Referring to the fact that you can actually edit the html markup too, not just the CSS. If course if you refresh the page, it goes back to how it was loaded from the server.