Speaking in broad terms, the Default and Light themes are in the very light greys and the Dark and black themes in the blacks.
The Custom theme, in its default setup, provides a mid-grey scheme (a bit on the darkish mid-grey, but that's ok).
PROPOSITION:
- Keep the Black OR the Dark theme
- Replace the theme not retained above by a formalized version of the (current) Custom theme and set this one as the default
- Keep the Light theme OR the (current) Default theme, but retain the name "Light" for whichever is kept
- Keep the Custom theme for those seeking to really drill down to something precise, but see below.
The darkness/brightness of the interface affects how images are seen, and, in theory at least, that interface should be mid-grey on balance (a range of greys are of course necessary for functionality, but they should all turn about a mid-grey). Indeed, a black interface can make a too-dark and/or under-saturated image look ok, and vice-versa for a bright theme. Thus, XnView MP should really be offering a mid-grey interface "out of the box" and as the default (with no need to go to the Custom theme).
However, this is an extremely personal thing, and thus it is very important to offer a very dark interface and a very light one, again, "out of the box," as well as a mechanism to hone in to one's personal preferences, hence the need to keep the Custom theme as well, but see below.
The current Black and Dark themes are too close to one another in terms of basic brightness: both of them are effectively "black" themes. So, there's no need for both. This is true as well on the other side of the brightness spectrum for the Light and (current) Default theme.
So, instead of having those redundancies, XnView MP should offer the "theoretically correct" mid-grey theme as a default, and easy choices for a black or near-black theme and for a white or near-white theme. Better still would be to offer Black, Dark Grey, Grey, Light Grey and White presets... but that's getting perhaps a bit out of hand.
Thus, for those seeking greater liberty in setting up their interfaces, the Custom theme can be kept. It is however, somewhat "out of reach" for the average user who does not have the markup knowledge to really make the most of it (me!).
So, another potential idea would be to steal what Affinity Photo does: Add a gamma adjustment slider for the themes. This would allow users who want, e.g. a dark-grey interface to choose the mid-grey and adjust its gamma down to taste.
Those two things are not mutually exclusive; there could be both a gamma slider for the "out of the box" presets AND the Custom theme as well.
So the dropdown menu at Tools > Settings > General would be simply:
Theme: Dark (or Black) [slider for gamma (greys-out for Custom??)]
Mid-grey (Default)
Light (or White)
Custom
As always, thank you for your consideration and happy shooting to all.