If you make a typo or regret sending a message, you can undo the action by enabling "Undo send." This setting gives you the option to take back a message you just sent.

To enable Undo Send:

• Click the gear in the top right
• Select Settings.
• Scroll down to "Undo Send" and click Enable.
• Set the cancellation period (the amount of time you have to decide if you want to unsend an email).
• Click Save Changes at the bottom of the page. If you had Undo Send turned on in Gmail Labs, your Undo Send setting will be on by default.

Note: To give you time to undo, Gmail delays sending the message for a few seconds. So if you don't select "Undo" within the time limit, your message will be sent. "Undo send" may not work if you experience connection issues.

Taken from this Google Support article.

You can't - if you are going to create your own custom themes.

Background pictures are currently only supported for themes that are either 1) by Google themselves, or 2) by authorized artists' themes submitted to Google.

If you are really dying to tweak the LooknFeel of GMail, you might want to give Firefox + GreaseMonkey a shot.