This is certainly possible, and in the mean time, you can actually achieve this with a workaround.
I just had the same thought like you and decided to research ways to make this work, I found the necessary info on here: https://developer.apple.com/documentation/appkit/nsappearancecustomization/choosing_a_specific_appearance_for_your_macos_app
All you need to do to achive this is to add this key/value pair to the Info.plist file which is stored inside the Dynalist.app file. To get there, simply right click the Dynalist.app and then click on Show Package Content. Once in there, navigate into the Content folder and open the Info.plist with an editor and add the key value pair. Obviously, it’s a good idea to backup the previous file before making the change.
<key>NSRequiresAquaSystemAppearance</key>
<false/>
Once made, restart Dynalist.app and the title bar should be in dark mode (if your system is set to dark mode).
Cheers, Chris
P.S.: The only issue is: if the Dynalist.app gets an update, this will have to be done again. But then again, the app is mostly a web app and most updates do not necessarily need an app update so this should work fine — for now.