I think both Designer and Photo are amazing applications, but I find I'm using them only sparingly because of the UI colour scheme issue. designerUK: The problem is aggravated with age (if you haven't noticed how your body changes with age, then I have to assume you are still a young person), but it's basically a scientifically proven thing that text and symbols with certain colour combinations are more difficult to comprehend than others, and this is regardless of age. There's a reason why (at least some) designers study this. If you want something to be read or understood easily and quickly, you try to make it as easy as possible for a majority of the users.
It has recently become popular to have colour schemes that lean toward darker tones. My favourite pet example of how it shouldn't be done is Pixelmator, where the combination of low contrast and rather smallish UI elements makes the experience a rather painful one (I'm 55, which might have something do to with it, your mileage may vary with age). I like the programme very much, but it's just too tiring to use for longer stretches of time. The UI in Affinity Designer isn't as bad as the one in Pixelmator, but I really would like to implore you to at least give us a selection of UI colour schemes. It is a well established fact that darker colours on a lighter background provide better contrast and therefore faster recognition and less fatigue. I hope you will be able to give us the same choices of interface colours that Adobe is providing in more recent versions of their Creative Suite. The beta is very impressive and I look forward to further betas and a well-rounded release version later on!