Thanks, Susan Otwell - it seems easy when you know how!

I have confirmed that the error is in my base_url. I can get the css to work when I replace the relative path <link rel="stylesheet" type="text/css" href="[(base_url)]assets/templates/tutorial/css/Style.css" /> with an absolute path in the tutorial template. My problem is that I can’t find where to correct the value of base_url which I entered incorrectly during the install. I’ve search the MODx manager, I’ve looked in config.inc.php - just can’t find where to correct it. Can you help?
Thanks, LarsB for your offer to look at my installation. I’m still a learner-developer, and am a little nervous of publishing the path to my installation on a forum. I’ve installed MODx a level above my domains, and I’m not sure of the implications of publishing the pathname, which includes the server name and my hosting account name. So until I know what I’m doing a bit better, I hope you understand my reluctance to disclose it. At the same time I appreciate your offer of help!
There are some errors in the wiki beginners tutorial
http://wiki.modxcms.com/index.php/Beginner%27s_Guide_to_MODx#Start_with_a_complete_functional_webpage which must frustrate a lot of beginners ... the html code references style.css and cixlogo-tropic.png, but when you follow the links to download these items, you end up downloading Style.css and Cixlogo-tropic.png (with first letters capitalized). Not much fun for beginners!
As per the first paragraph, my remaining issue is where to correct the value in base_url variable? Thanks, everyone!