Since you asked nicely enough, I'll try to give you a solution.
There should be an [Options] at the top of the page. When you click on it, the box will appear with User CSS in it. Click on User CSS, and you'll see the following comments in the box where you can paste in your own custom theme:
/* Enter here your own CSS rules… */
/* If you want to make a redistributable style, be sure to
have a Yotsuba B theme selected. */
/* You can include CSS files from remote servers, for example: */
/* @import "http://example.com/style.css
1) That tells you the theme you should have selected in General when using custom CSS is Yotsuba B because customizations are oftentimes written around/based on the default. You can play around with it and try the other built-in themes though of course.
2) The fauux theme you are referring to is https://www.lainchan.org/stylesheets/fauux.css
, no? So I do what the example shows me and paste in the following line uncommented.
Alternatively, you can go to that URL where the theme is stored, select all, and copypaste that directly into the field.