WP is flexible and you should know what the CSS or style sheet does and is.
You can reset defaults or customize just a certain segment.
Or, as drag mentioned, just edit the HTML vs Visual editor when you are creating a page.
Wordpress, in my opinion, is pretty much a basic HTML site broken down into components.
No matter how a site is constructed, if you want to change something you have to know where in the line of code it is and how to change it.