Styling — such as the type of font, font size, and width of text — for a Quartz v4 website is defined under quartz/styles.

For example, to change the width of the main text box to resemble the default view in Obsidian, you would navigate to quartz/styles/base.scss and edit the following lines:

.page {
-  max-width: calc(#{map.get($breakpoints, desktop)} + 300px);
+  max-width: calc(#{map.get($breakpoints, desktop)} + 200px);
  ...
}

So far haven’t done much changes, apart from the aforementioned one.