dark theme is basically just to remove the bright blueish color theme from the default theme. i prefer content on white background on the desktop. amoled was created by a mod here who wanted pure black for his phone. bold and normal white text would have visibility issues on black background so there's that. for best experience, native blue theme is best because all color coordination is tested on it. on amoled and dark themes, sometimes colors do not work well because of how hard it is to get it right.