So in your administration panel, under “design” is “theme editor”.  At the top of the Theme Editor is a “Select Theme To Edit” title and a drop-down box.  Select your current theme.  If you’re not sure what it is called, go to the Design>> Themes tab, at the top is “Current Theme”, including a screenshot and some information including the title.

Back to the Theme Editor.  Select your current theme from the drop-down menu.  On the right hand side is a whole list of templates.  At the bottom of that list, there is a subsection, titled, “Styles”.  Select the main style sheet.  It is usually titled, “style.css”.

Now, instead of rewriting the styles online, I highly recommend selecting the text with your mouse, and copying it.  Paste it into a Text Editor(NOT a word processor like Word).  Crimson Editor is an excellent one that is available off the internet, and is open-source (free).  Once you’ve gotten those styles on your own computer, in said program, hack to your heart’s content.  When you’re finished, select the styles, copy them.  Go back to the Design tab>>Theme Editor>>current theme>>styles>>style.css.  And paste it right over the old one.

Refresh your page to see what you’ve done.  Oh dear.

Go back to your text editor, which is still open, right?  Click on undo until you’re your fingers fall off.  Select and copy the original styles.  Go back to your once-beautiful-blog, Design tab>> Theme Editor>> current theme>>styles>>style.css.  And paste the original code overtop of your shiny new code.  That should put you right back again.

This is the easiest way to avoid making irreversable mistakes.  Or you can do what I did: make the changes online, which is cool because you can see the changes live as you write it.  And then do something that you can’t remember how to reverse.  Something really aweful.  Then all you have to do is find that darned theme on your hard-drive, the one that you uploaded a year ago, and upload a brand new fresh style.css.  And begin again. Uggg.  Do it the first way, trust me.