As of the 4.6 update, Boxer supports HTML signatures for your outgoing messages.
To turn it on:
1. Tap on 'Settings' in the lower right corner
2. Select the account you would like to change your signature
3. Tap on 'Signature'
3. Switch the "HTML" toggle to the On position.
4. Type in the name or text you would like your signature to display, and then highlight it by long-pressing and tapping on "Select".
5. From the menu that pops up, tap on the arrow pointing to the right.
6. Tap on the rich text options (Bold, Underline, Italicize).
7. Select whether you would like the text to be bolded, underlined, or italicized.
8. Once you have finished, tap on "Save" to save the new signature.
A How-to video can also be found here: https://www.youtube.com/watch?v=mI7LeflvYVM