Theory is practical. A knowledge of database management fundamentals enables you to identify and understand relevant issues, gives you the toolkit you need to solve day-to-day problems and allows you to communicate with your peers using a common vocabulary.
In my experience the internet is rarely a good place to gain basic knowledge because there is too much rubbish and misinformation about.
There are some good book recommendations listed here:http://www.dbdebunk.com/books.html
I also recommend Terry Halpin's book "Information Modeling and Relational Databases" if design theory is specifically what you are interested in.
