Xubuntu (Ubuntu) 16.04, Nvidia GTX 720
In a newly opened application of Blender (mine opens in Eevee), when changing settings for the background (aka color/strength) you have to disable Use Nodes and then re-enable them to get it to update. Also happens in other engines, but you will have to play around with it because I can't quite tell how it's doing it. After adding a mesh and doing some stuff it works. No idea how.
Open Blender. Go to the 'World' tab. Click 'Use Nodes'. Try to change either color or strength (nothing should happen). Disable 'Use Nodes'. Re-enable 'Use Nodes'. It should update now. Play around with this in other engines and such - I am not quite sure how it randomly starts to work or works differently in other engines.