Allow for overriding styles in a stylesheet held in the configuration directory

[davep]

Frogmouth's code holds all of the styling in the code, mostly in DEFAULT_CSS on the relevant widget, with a bit over "overall" styling in DEFAULT_CSS on the main screen.

It would be useful, however, if an end user, with some knowledge of the styling options, could override those styles in a stylesheet of their own; but not one that is shipped with and as part of the application, instead one that is optionally held outside of the installation directory (most likely held in the configuration directory -- ~/.config/textualize/frogmouth on most systems).

I need to check if this can be done as part of Textual's current stylesheet loading system (is it possible to attempt to load a stylesheet that doesn't exist without that being an error?), and if not perhaps explore the idea of adding such support to Textual.

[davep]

See also #57