1.
Home Screen Shortcuts - BlackBerryFAQ
This ^^ should apply to the Bold. Some shortcut keys do vary on the Bold. Experiment and let us know.
2. The sig? Are you on BES? If on BIS:
How do I edit the "Sent from my BlackBerry®..." message? - BlackBerryFAQ
What is your default email issue? You do set your default "sent from" at Options > Advanced > Default services. NOT ALL email will default from the 'sent using'. If you are replying to an email, the BB will ALWAYS reply from the address from which it was received.
And, the BB will always default to "send using" from the last email account you sent an email to that contact. It remembers the last one you used for that contact. So, if that is your issue, make sure you change that on any that are sending from the wrong account, and the BB will remember.
3. Options > Themes. I know 'Today' is one, "Dimension Today" is another, but I am not familiar with what all themes the Bold includes.