(cherry picked from commit 946159aee0beb36889c566416e02ad7fddd69df5)

See https://developer.gnome.org/hig/reference/keyboard.html

Also moves CSS bits to a resource of its own.

See
https://gnome.pages.gitlab.gnome.org/libadwaita/doc/main/css-variables.html

Thus, users are greeted with all graphs visible.

Also sorts by CPU by default now. These changes will arrive anyway with
the port of the processes list to GtkColumnView.

load_settings is already called a few lines below.

This reverts commit 468f81ac.

(cherry picked from commit afa4baacf1dedda5627df6df4d1ecdd28b652606)