Its alright, personally I almost always prefer a dark mode, so the "modern clean" white colour. I also think that whatever app, website or application you are developing you should always consider doing a dark mode. Not only is it easier on the eyes, but it generally saves battery life, specifically on phones. I almost exclusively use dark mode whenever possible, and when its not I tend to look to force dark mode.

What's the sliding option next to the "boards" dropdown used for?

I think this could be suggested to the new forum software Epochtalk. As I see there are already some suggestions for the new UI like this one https://bitcointalk.org/index.php?topic=5296215.0
I personally like the old one, or maybe more accurate is to say I'm used to it but I would love to sea "night mode" implemented as Welsh mentioned above.
Overall it's a nice design but it won't be implemented to the official forum anyway.

Pretty clean minimalistic design, I like it so far.

Do you have any other splash screens made for say, how a thread would look in this particular UI, and would you allow for users to be able to change theme, color, font etc?

Also, there is no mention of the app being related to bitcointalk or a bitcoin forum in general, or even a logo for that matter, so perhaps that could use.

great work but it's a little too bright for me and hurts my eyes(my eyes are sensitive to bright colors). maybe adding darker colors in the UI might reduce its brightness and help those who have color sensitivity to look on the UI for longer times.

Darker version looks much better for me, and maybe you can move this image to first post also along with light version.
Best thing would be to have simple switch so anyone can choose what they like, and I think app could attract younger generation in forum.

Me too--I can't stand too much white on anything, including websites, walls, cars, and sclera.  The dark mode and the overall look of what OP created it pretty damn good if you ask me, and if I ever accessed bitcointalk on my smartphone I'd be happy to have the site showing up like that.  

I used to visit the forum a lot on my phone, but I don't anymore--but when I did, the lack of a modern mobile-friendly site used to drive me up a fucking whitewashed wall.  I (and a lot of other vocal members) could never understand why Theymos hadn't kept up with the times in that respect.  Yeah, I get that the new forum software should be arriving any day now, but bitcointalk has been mobile-unfriendly for as long as mobile-friendly sites have existed.  Think about that.

I kind of agree with you, although the image you showed is kind of overused.  It looks like Mycelium's, Reddcoin's, Curecoin's, and I'm sure tons of other coins and assorted blockchain projects' logos, i.e., it's very unoriginal. 

However, the benzene-sniffing part of my brain really likes it.

I would definitely use the darker version but tilting my phone to the side ^{[landscape mode]} would result in having a small middle section ^{[due to the header and footer heights]}...
^{- I would love to also see a responsive design in landscape mode.}

I have low hopes for forum to implement anything like this, but anyway,

UI interface is beautiful, the design is streamlined, but there are a lot of notes:

 - There is no need for there to be a stats icon.
 - There is no notification icon or at least icon for "Show new replies to your posts."
 - There is no need for Help icon.
 - Instead of a member icon, make it an icon to search for members.
 - add Create topic.

Overall, I liked the design, especially the dark mode.

I prefer the dark one, because with the white one, too many things blend together.
This might be just me, and i'm  aware it's subjective, but i like clean lines and separations in an app