I just assumed that this package was only meant to change the color scheme of the IDE, without additional tweaking of the interface. So the goal is to take the native look and only change the colors to dark. Meanwhile, currently, not only is the color scheme incompatible with the native appearance of the interface, it also does not comply with the dark theme contrast guidelines (the contrast is too high) and permanently modifies component settings, such as expand sign in all TTreeView controls (instead of native |^| are set to |+|).
So please, try to immitate the native theme by changing the color scheme only, keeping everything else in the default style, alignment etc. If you need inspiration, just make a screenshot of the standard (white) Lazarus IDE, and then invert colors in the Paint or any other graphics editor. Then slightly adjust the lightness and voilà — you have all dark shades ready to implement in the package.