main: Always pass a default stylesheet when constructing the theme

main: Always pass a default stylesheet when constructing the theme

There is not always a clear distinction between code and style,
which is why the interface ends up being mostly unusable when we
end up without *any* style, for instance because the specified
application-stylesheet is corrupt.
Setting the default stylesheet in addition to the application-stylesheet
is no guarantee for non-default themes not messing up the interface, but
it should at least lower the risk ...

https://bugzilla.gnome.org/show_bug.cgi?id=700097