Created attachment 243807 [details] [review]
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 ...

Review of attachment 243807 [details] [review]:

Makes sense.

Attachment 243807 [details] pushed as 990f683 - main: Always pass a default stylesheet when constructing the theme

The default theme cause that we need to override all properties in a custom theme. As for example:

If I create a theme with this style class:

.popup-separator-menu-item {
    -gradient-height: 2px;
    -gradient-start: rgba(85,85,85,1);
    -gradient-end: #555555;
    -margin-horizontal: 1.5em;
    height: 1em;
}

Like the shell have this style class in his default theme:

.popup-separator-menu-item {
  height: 1px;
  margin: 6px 64px;
  background-color: transparent;
  border-color: #343a3a;
  border-bottom-width: 1px;
border-bottom-style: solid; }


The property margin of the default theme will survive and thats it's not wanted, because the new definition have all that it's needed to show the separator.

This create a conflict on both style class, forcing the custom style class to override all properties of the default one...

What i considered a moderate behaviour it's do the same but at the style class level, not at the level of the properties of the style class.

PLEASE SEE: What could happen, when also there are an error on the default theme?

    margin: 6px 64px; it's of course an error. 64 it's to much...

https://github.com/GNOME/gnome-shell/blob/7dd6b7f04f8619bb766e102802efcd0e41454015/data/theme/gnome-shell.css#L532