Have a look at the CSS snippets link above. You don’t write the CSS in the body of a note; you create a .css file and put the CSS snippet(s) in there that then affect the style of your note, the Obsidian interface, etc.
If you enabled that snippet in the default theme, anything with the variable of --text-normal would turn red (which includes regular note text and a bunch of other stuff). Probably don’t want to do that.
Looking at the third css file in your image, maybe you are trying to change the readable line length of the editor? You can use either of these with the default theme:
body {
--file-line-width: 60rem;
/*--file-line-width: 900px; /* default is 700px */
}
The steps to set up the Dashboard are all in the medium link here: