Replace GtkStyle usages with GtkStyleContext

This makes evolution depend on theme-defined named colors, namely:
   theme_bg_color
   theme_base_color
   theme_fg_color
   theme_text_color
   theme_selected_bg_color
   theme_selected_fg_color
   theme_unfocused_selected_bg_color
   theme_unfocused_selected_fg_color
If it's not defined, then a fallback color is used, in the worse case
one of the fallbacks defined in evolution itself.