The message is either, “You must indent.” or, “You don’t have to.”
That’s correct. CSS ignores whitespace.
This:
.post-main {
font-family: Ubuntu,sans-serif;
color: #222;
font-size: 1.1em;
}
is every bit as correct as this:
.post-main{font-family:Ubuntu,sans-serif;color:#222;font-size:1.1em;}
Use whatever works for you.
(The second example is minimized—all the white space has been removed. There are plugins that will do this for, and if you have very long stylesheets, it will save some overhead on your server. That may explain where you have seen examples of this.)
Until Automattic decides if it will adopt CSS or not I will keep my code to a minimum.
All themes use CSS. Whether or not it’s minimized is up to the theme designer. When you are creating a child theme or adding your own CSS via a custom CSS plugin you are perfectly free to do whatever works for you.