If you want to change the font, colors, width of a column, etc, you can do this on your own computer, where there is an undo button.
In your administration panel, under “appearance” 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 Appearance>> Themes tab, at the top is “Current Theme”, including a screenshot and some information including the title.
Go 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”.
Refresh your page to see what you’ve done.
Go back to your text editor. Click on undo repeatedly. Select and copy the original styles. Go back to your 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 irreversible mistakes.
*warning: Playing with your theme has the potential to ‘break’ the theme editor. In that case you have to use the host’s file manager or FTP to upload your original copy.