The reasons for a light theme are photographic and physiologic.
I use a print workflow with a dim screen (80 cd/m2) and a white background. Viewing images on a dark background makes them look bold and vivid, but this is deceiving; they will not look like that printed and mounted on a white board. If you make them look good on a dim screen with a white background, they will look good printed. (“My prints are too dark!” “No, your screen is too bright.”)
And for photo editing, the theme should consist of neutral colors; vividly colored window elements are distracting, and may distort your evaluation of the image colors. Some people even paint their digital darkrooms gray.
A light screen constricts the iris and improves focus; when your vision starts to go you will understand this. (Presbyopia means “old eyes.”) Light text on a dark background seems to expand and become fuzzy, and is harder to read. If you do a search you will find differences of opinion on this, but note that a significant population is of the clear opinion that a light background is less strain on the eyes. That is why operating systems and so forth provide different choices for theme colors. And the people who say a white background is a strain may be using bright high-temperature factory screen settings, which attract people in showrooms but are terrible for everything else. Note that some recommend a slightly muted or even yellowish background for text on a light theme.
Using the 5.0 RawTherapee theme, I can barely tell whether the clipped highlight and shadow indication buttons are selected, for example, and the light-gray-on-dark-gray text is very hard to read. The TooWaBlue icons are a little better but the text is the same, and the vivid blue sliders and borders are distracting. And these are the only two choices I have; there is no system theme checkbox.
Having said all this, those who like the dark themes are welcome to use them; but please put back the system theme box; and it would not hurt to provide some light neutral themes as well.
Thank you . . . .