Unless a game studio develops its own engine, developers are usually constrained by the customization capabilities of the underyling game engine they use. So any in-depth graphical customization can only be offered (with reasonable effort), if the game engine exposes that setting internally.
To my knowledge, Nimbatus is being developed with the Unity Game Engine (https://unity3d.com/), so most graphic settings are ultimately determined by that engine's capabilities. As far as I know, Unity allows to set the target framerate, but I'm not sure whether that achieves an FPS lock as requested by the OP. At least there might be a way for the devs to implement it.
With regards to the windowed borderless mode, this is indeed supported by Unity, though it can only be enabled via a command line argument called
-window-mode borderless
. To enable borderless mode in the demo, create a shortcut to the Nimbatus.exe and add the arguments to the target field of the shortcut's properties (right click → Properties). Unfortunately, Unity doesn't seem to offer an official way to switch modes while the game is running, so there is little the devs can do about that.
