How can I adjust the Home Page skeleton so that when the hero image is loaded on mobile, the focus point is the left side of the image, not the center?
This is theme dependent, please tell us which theme you are using? You might be confused, it’s not related to the skeleton.
Basically you will need to modify the theme template Twig file. There are several methods for modifying themes including copying, subtheming, and direct editing.
Your request also seems like more of an HTML/CSS/responsive design question, which is not really Grav-specific, so you might have more luck posting this question as a repository issue for your theme.