Read the Directions

I almost messed up my entire site this morning.  I was trying to update the theme of the blog to a more recent version, and I couldn’t even access the admin panel after uploading the current version.  It turns out that if I had read the directions included with the updated version, I would have known better. 
I’ll update it later, but I was really concerned that I had done something horrible for a little while.  I deleted the new theme from the themes folder, and everything returned to normal.  I guess this is another lesson that I shouldn’t take anything for granted… 
Why do some people say the phrase “take it for granite”?  Am I wrong, or are they?