fix(display): Imply LVGL mono theme when display is enabled

All displays currently used with ZMK are monochrome so it makes sense
to enable the mono theme by default, which can be disabled by the user
since we use the "imply" statement.

Without this theme setting, the small font size selection for widgets
at the bottom of the stock status screen does not work.

app/src/display/Kconfig

@@ -9,6 +9,7 @@ menuconfig ZMK_DISPLAY
     select LV_THEMES
     select LV_THEME_MONO
     select LV_CONF_MINIMAL
+    imply LV_USE_THEME_MONO
 
 if ZMK_DISPLAY