themes: Define style for textview.view

Resolves #14617
Previously, we had defined the Default theme for
GtkTextView text with "textview" and "textview text".
However, the Python console also outputs some text
that is formatted by "textview.view". Since we did not
define a style here, it was possible for a System theme leak,
resulting in unreadable console output (separate from console
input).
This patch should resolve the issue by adding textview.view to
the GtkTextView definitions in common.css.

themes/Default/common.css

@@ -123,7 +123,7 @@ stack {
 }
 
 /* GtkTextView */
-textview text, textview {
+textview, textview.view, textview text {
   background-color: @extreme-bg-color;
   color: @fg-color;
 }