Post width – visibility of long lines

Please, is it possible to enable the (Discourse) Default theme?

I have a user style that increases visibility at sites elsewhere (for example, the first screenshot below).

Unfortunately, this widening is not effective at Unix Linux Community with themes such as Graceful Wide (second shot).

Postscripts

Minima can be widened, however readability is reduced by use of a serif font.

Dark and Light can be widened.

Now, I wonder which theme was preferred before I tested Graceful Wide – I switched to this, temporarily, only because (unless I'm mistaken) I didn't get the widening with my usual preference.