The theme is only providing a space for your video to appear, and not configuring the player for you unless specifically intended as a theme feature (Ambiton, MotionPicture).
Videos embedded using oEmbed or embed codes will start in the default quality unless you A: use specific HD variables in the embeds or oEmbed link or B: the theme supports HD conversion.
First, check your theme options panel in the post editor for a Quality menu (MotionPicture and Ambition, for example) and select HD.
If your theme does not have an HD selector, you will need to tailor your embeds/oEmbed links. You may also be able to set the default from your profile on your video host.
The key is the “?vq=hd720” at the end of the video URL in the embed code. Example:
<iframe width=”560″ height=”315″ src=”http://www.youtube.com/embed/_3rLN6WR3oA?vq=hd720” frameborder=”0″></iframe>
Vimeo requires you to set HD defaults in your profile, which may require Vimeo Plus.
View the Vimeo HD FAQ