The first picture shows how to access the tool "inspect element". Simply right click on the part you want to see the CSS for, in this case the remix wiki box, and select the "Inspect element option.
The second, third and fourth pictures show the lines that refer to each of the boxes you asked about in the forum. There is more detail about each of these boxes, which can be accessed by clicking the chevron at the left/beginning of the line, expanding the menu down.
The fifth picture shows an expanded view of the tool. It can be expanded by clicking and dragging the top of the page split - the bar above all the icons labelled element, resources, network, etc. The red outline shows the CSS elements individually. If you are having trouble finding an element you can view the page code itself by clicking on the link in the top right of one of the smaller boxes within this element menu. The normally have names along the lines of oasis.scss (this isn't the full name) and are underlined and grey. This opens a tab within this inspect element menu, although I prefer to right click the links and open them in a new tab. You can them use Ctrl + F to search for the element you are looking for.