Very nice! Feedback below:
This is more "demo site" feedback but might be something you can handle in the theme itself, but on posts in Masonry layouts, sometimes it's not clear whether the entry header div belongs to the post above it or the post below.
Take this example.
Is the "The Beginning of WordPress" post's featured image the dude, or the starry night sky? I know the answer is the dude, but it's something a fleeting visitor might not realize and get confused by.
This is kind of an inherent "issue" with Masonry, but I'm wondering if anything could be done about these blank spots with the black background.
I tried messing with a different background color, but that made it look more off. Maybe a subtle background pattern on top of the black background would better signify the difference between black backgrounds of entry headers and black backgrounds of empty space? Not really a big deal, just a thought.
Wondering if it might look a little better to use (extra) padding, instead of margin on the entry headers, so it dark->transparent gradient looks like it blends in directly with the entry header.
Again, not a big deal...just a thought.
Wonder if it would look better if the background of the mobile category menu was the same yellow as the non-mobile menu.
Instead of black.
The slider behaves weirdly at mobile breakpoints.
The mobile category menu appears in between the featured image and the content. When expanded, the menu items are hidden behind the content.
Sometimes, the menu doesn't appear at all. And the content from the slider covers up the featured image from the first normal post below.
Also, there isn't any way to navigate between slides on mobile besides just waiting for them to switch naturally.
Noticed these issues in both Chome and Firefox on macOS.
It's cool that the post navigation lines up perfectly with the footer widget columns.
This is just "demo site feedback" but I couldn't find any threaded comments. I also didn't look through every single post so maybe they're there somewhere.
I know it's not super common, but showcasing what comment navigation would look like too might be helpful for those who run sites that get a lot of comments and choose to paginate them.
Comment form looks good though.
I like the breadcrumb feature, but only see it on pages. Wondering if it might be useful on other types of pages too? Like archives, single posts.
But that may be getting too deep into "plugin territory" and not sure if it would be allowed by .org standards.
More demo site feedback, but it looks like the LaTeX plugin isn't working.
Seen on the HTML elements page.
I noticed you're using Genericons to display the social menu icons. Not sure if you're aware, but Automattic deprecated social icons in the latest version of Genericons and spun them off into their own icon font called Social Logos.
I couldn't find any other place you were using Genericons (the search icon looks to be a direct SVG) so it might be a little more efficient to use Social Logos instead.
Anyway, again, very nice!
The mobile slider is the only really major issue I found. Everything else was just random thoughts that I thought were worth mentioning, but not a big deal to fix.