feat(display): Add setting to invert display colors

Add CONFIG_ZMK_DISPLAY_INVERT Kconfig to invert colors
(black-on-white to white-on-black) on monochrome screens.
Currently applies only if CONFIG_LV_USE_THEME_MONO is selected,
which is the default unless user overrides it.

app/src/display/Kconfig

@@ -15,6 +15,13 @@ config ZMK_DISPLAY_BLANK_ON_IDLE
     bool "Blank display on idle"
     default y if SSD1306
 
+if LV_USE_THEME_MONO
+
+config ZMK_DISPLAY_INVERT
+    bool "Invert display colors"
+
+endif
+
 choice LV_TXT_ENC
     default LV_TXT_ENC_UTF8
 

app/src/display/main.c

@@ -91,7 +91,8 @@ int zmk_display_is_initialized() { return initialized; }
 static void initialize_theme() {
 #if IS_ENABLED(CONFIG_LV_USE_THEME_MONO)
     lv_disp_t *disp = lv_disp_get_default();
-    lv_theme_t *theme = lv_theme_mono_init(disp, false, CONFIG_LV_FONT_DEFAULT);
+    lv_theme_t *theme =
+        lv_theme_mono_init(disp, IS_ENABLED(CONFIG_ZMK_DISPLAY_INVERT), CONFIG_LV_FONT_DEFAULT);
     theme->font_small = CONFIG_ZMK_LV_FONT_DEFAULT_SMALL;
 
     disp->theme = theme;