Hmm – it looks like the Neuton Regular font isn’t being picked up on mobile either — I see what looks like Times or Times Roman.
In terms of the font size modification, I’m not sure why it’s not being applied, as the Customizer CSS should apply to the site, regardless of screen size.
Could you try temporarily changing the colour of your site title to confirm that the CSS really isn’t being picked up? It’ll be more obvious that way. So change what I gave you earlier to this:
.home .entry-title {
font-size: 49px;
color: #b0b;
}
If your featured post title on mobile does not turn purple we know the custom CSS somehow isn’t being applied.
The next troubleshooting step I’d suggest at that point is to deactivate all your plugins temporarily to make sure there isn’t one interfering with your font and custom CSS on mobile. If the problem goes away, reactivate them one by one to find the culprit.