This is really a very complicated issue…
I am disabled myself, I can only access my PC via an eyetracker (which unfortunately only works on windows systems btw). I access nethserver either via the GUI or via ssh respectively directly (if I am working on a virtual box).
and I have some experience in making an accessible website - I am the webmaster of https://chronicallyacademic.org - however I am not an real expert…
Regarding the html/php code, the GUI looks pretty accessible (of course, one could improve it - e.g., by using aria tags and so on… However, each field has already a label and the items have title tags etc.).
But what can be improved (and this is probably rather simple to do) are the contrasts… I have just analyzed a few contrasts in the GUI, using the contrast analyzer provided by https://www.paciellogroup.com/resources/contrastanalyser/… and I found, e.g., that the background color of the selected menu item should be much darker (I mean .Navigation.Flat a.currentMenuItem of which the background color is defined in base.css l. 49) or alternatively the color of the respective font should be changed to #000. this is certainly not the only color that can be improved…
Furthermore, this community is not accessible at all…! 
Even I have already encountered several problems in using it (and I am not blind)… however, I know how difficult it is to make a community platform accessible (I am currently trying it for the website I mentioned above… When/if I am finished, I will of course share the code on github and post the link here…). The thing, I personally am struggling most with in this community platform here, is that it is not so easy to scroll the posts in a topic, i.e., when using the page up/page down keys on the keyboard, jumps often occur (especially if images are posted etc.). I don’t know why this is happening…!
Of course, there are much more things to do if we want Nethserver really accessible for everyone… This would be really really great, of course, but it will be also very hard…!