Would a weapon locker be out of place?
I also picture a large screen that displays maps - sector, system, planetary - although this could be built into the picture 'window' or the desk top.
Also I assume this office is within atmosphere, otherwise there'd be a Vacc suit.
How 'loyal' is he to "superiors" (past or present) from ex-commanding officer to Strephon? I'm thinking of all the pictures of the Queen I can think of in prominent places in rooms I've visited in different parts of the world. Picture, statue, hologram...
I could also imagine some very subtle wall markings along the lines of a lot of small crosses. If you look very carefully you'll see names on each one. Again, could be 'electronic' - perhaps instead of 'me wall' type photos, the crosses are accompanied (or replaced by) pictures of faces. Maybe a lot and quite small. And could be subtly moving as they're actually 'video'[1].
if he's *that* spartan! what about not having painted/decorated walls at all, but just the fabric of the building. Kind of making the statement 'I don't need any fancy frills, just the necessaries'. With the added implied 'how tough am I?'
tc
[1] (Like the Harry Potter wall portraits - I saw this kind of thing used *very* effectively on my tour of a chocolate factory. In one room, there were some digital pictures of past figures from the company. As the tour guide spoke, you suddenly realized they weren't static pictures at all, but very subtlety moving - as they sat at a desk or posing for the camera on a seat or whatever. Even better, the tour guide would then interact with them in turn! Very clever - they must have practised their timing endlessly).