Fix theme changes only taking effect on the next load

[cyberhirsch]

dt_gui_load_theme() created a new GtkCssProvider on every call and added it to the screen. gtk_style_context_add_provider_for_screen() takes its own reference, so the g_object_unref() at the end never removed it and providers accumulated, all at GTK_STYLE_PROVIDER_PRIORITY_USER + 1. With several providers at the same priority the oldest one still won the @define-color lookups, so selecting a theme appeared to do nothing until the theme was loaded again -- switching to another theme and back was the usual workaround.

Create the provider once and replace its data on subsequent calls.

While here, the theme selector in preferences silently fell back to list index 0 when the configured theme didn't match any entry, so an unknown or empty value was displayed as whichever theme happened to sort first rather than as the actual default. Fall back to the configured default instead, and free the per-entry name string, which the combobox copies.

[TurboGit]

With several providers at the same priority the oldest one still won the @define-color lookups, so selecting a theme appeared to do nothing until the theme was loaded again

Can you clarify? And a way to reproduce because I don't see that on my side. TIA.

[TurboGit]

@cyberhirsch : Just a gentle ping for my previous question. TIA.