I love the minimalistic approach of this theme and I use it for my blog.
The only issue I have with it is that on the mobile version the menu is compacted and only the word "MENU" appears. I did some informal user testing with friends and noticed that people don't see the menu (they just don't tap it because nothing is indicating it can be tapped).
I solve the issue adding a little arrow to the "MENU" label:
<h1 class="menu-toggle"><?php _e( 'Menu ▾', 'ryu' ); ?></h1>
I don't know if this is the best way to solve this usability issue, but it is at least a quick one!