The dark background that you are referring to is the OBC background. It can only be activated by purchasing an OBC subscription. When you buy OBC you can set the dark background to appear on all pages, and users will always see your profile with the dark background.
First, you log onto Roblox. Next, you click "My Roblox". Finally, click "Profile", and your there!
For your profile, you cannot change it to a specific background (currently). You can change the theme though, you can change it to Roblox 1.0, 2.0, 3.0, or if you are a member of Outrageous Builder's Club you can change it to Outrageous 1 or 2.
Go to my roblox and select account
You have to buy Builders Club.
You can't.
roblox> account> go all the way down> write in the box, to see your blurb, go to your profile, then view public profile
Go to profile in MY ROBLOX or Places and press the build button.
To write things under your name in Roblox, you can use the "About" section of your profile. Go to your profile page, click on the "Edit Profile" button, and then you can add or update your description. This description will appear under your username on your profile. Make sure to follow Roblox's community guidelines when writing your content.
Open ROBLOX studio, Find your profile, the edit button is on your places.
Click on My Roblox, then click on profile, scroll down until you see stats.
Go to your profile, click Build. Oh, I forgot. You need a Roblox Studio to do that. Then you build.
It is located under account.