Added example theme (#161)

Still to do is move the default theme files and values to this new `themes` folder.

Author: ducalex

THEMING.md

@@ -28,6 +28,7 @@ static struct
         rg_color_t item_disabled;
         rg_color_t item_message;
         rg_color_t scrollbar;
+        rg_color_t shadow;
     } style;
     char theme_name[32];
     cJSON *theme_obj;
@@ -173,6 +174,7 @@ bool rg_gui_set_theme(const char *theme_name)
     gui.style.item_disabled = rg_gui_get_theme_color("dialog", "item_disabled", C_GRAY);
     gui.style.item_message = rg_gui_get_theme_color("dialog", "item_message", C_SILVER);
     gui.style.scrollbar = rg_gui_get_theme_color("dialog", "scrollbar", C_WHITE);
+    gui.style.shadow = rg_gui_get_theme_color("dialog", "shadow", C_NONE);
 
     return true;
 }

components/retro-go/rg_gui.c

@@ -0,0 +1,53 @@
+{
+    "description": "An example theme to show how themes are made",
+    "website": "https://github.com/ducalex/retro-go/",
+    "author": "ducalex",
+    "dialog": {
+        "__comment": "This section contains global dialog colors",
+        "background": "0xFFEF",
+        "foreground": "0x0000",
+        "border": "0x94B2",
+        "header": "0x0000",
+        "scrollbar": "0x0000",
+        "shadow": "none",
+        "item_standard": "0x0000",
+        "item_disabled": "0xF8B2",
+        "item_message": "0x4208"
+    },
+    "launcher_1": {
+        "__comment": "This section contains launcher colors variant 1",
+        "background": "0xFFFF",
+        "foreground": "0x0021",
+        "list_standard_bg": "transparent",
+        "list_standard_fg": "0xF8B2",
+        "list_selected_bg": "transparent",
+        "list_selected_fg": "0xF800"
+    },
+    "launcher_2": {
+        "__comment": "This section contains launcher colors variant 2",
+        "background": "0xFFFF",
+        "foreground": "0x0021",
+        "list_standard_bg": "transparent",
+        "list_standard_fg": "0x2444",
+        "list_selected_bg": "transparent",
+        "list_selected_fg": "0x07E0"
+    },
+    "launcher_3": {
+        "__comment": "This section contains launcher colors variant 3",
+        "background": "0xFFFF",
+        "foreground": "0x0021",
+        "list_standard_bg": "transparent",
+        "list_standard_fg": "0xF8B2",
+        "list_selected_bg": "0xF800",
+        "list_selected_fg": "0xFFFF"
+    },
+    "launcher_4": {
+        "__comment": "This section contains launcher colors variant 4",
+        "background": "0xFFFF",
+        "foreground": "0x0021",
+        "list_standard_bg": "transparent",
+        "list_standard_fg": "0x52AA",
+        "list_selected_bg": "0xF800",
+        "list_selected_fg": "0xFFFF"
+    }
+}