Years ago we had coloured icons on mac, then grey icons on a light grey background. It's maybe very "digital" but my eyes, as all human eyes, react sluggishly to differences in brightness. But not regarding colors.
This is the trick which was used for PAL system (on TV) in comparison to NTSC. The phase shift, which result in changing colors is the result of NTSC an people realise some color differences, when faces become mauve or lila. The PAL system replace the phase shift in luminance difference, because the human eye react tardy for luminance differences.
Reducing the distinction at luminance differences (means brightness) will keep the brain working hard, which result in tiredness.
I don't understand how a professional tool like Affinity Designer (which is mostly used from professionals) offer a such GUI!
Please, please, offer a brighter GUI or configurable!