I think i saw it one of the earlier releases.
Lately i started rigging some characters for a animation in 2.8 and i kinda wished the bones could be set more transparant is there an option in the gui or in the theme where i can set that ?.
I’d like them be be just barely visible, so i can judge overal visual apeal of the scene instead of seeing mostly the bones.
I remember this too. Here’s a screenshot of when I was playing/loving it.
This part of the announcement video shows the option (55 seconds in):
What happened? 
Please tell me it’s not gone (or coming back soon).
I don’t know if this helps you since I don’t know how to make bones transparant but this is what I usually do:
Turn on X-ray but set maximum draw type to Wire. This will allow me to still see, select and pose the bones without them becoming too obtrusive. 
I know of wire but thats to much noise in 3d drawing when i am posing.
I don’t think there are additional settings to tweak opacity. You can turn it on or off and that is it.
I agree an option for opacity would be nice. Heck, a shadeless option would be great.
Or, even cooler would be something like this:
I still have to try if something like this is possible in blender but I am a little worried about the impact on performance. Here is a short movie on how to set a control scheme like that in Maya:
PS
Just to be certain, are you aware of “bone shapes”? You can use your own shapes/models for bones. Most rigs use these for all their control bones and hide all other bones. Case in point: these are all the control bones visible for a foot on one of my rigs.
That’s general foot controller, heel, toetip and toes. Not that distracting imo
I remind that one time in development (and published videos) it was possible to set the bone tansparancy percentage.
If it would be possible to have that again (by python?) or some gui slider then i be happy.
