add theme inheritance, fix broken theme when no longer installed

fixes #74

assets/romfs/themes/abyss_theme.ini

@@ -2,8 +2,7 @@
 name=Abyss
 author=TotalJustice
 version=1.0.0
-; unused currently
-preview=romfs:/theme/preview.jpg
+inherit=romfs:/themes/base_black_theme.ini
 
 [theme]
 background=0x0f111aff
@@ -12,11 +11,3 @@ selected=0x0f115cff
 selected_overlay=0x529cffff
 text=0xffbc41ff
 text_selected=0x529cffff
-
-icon_audio=romfs:/theme/icon_audio.png
-icon_video=romfs:/theme/icon_video.png
-icon_image=romfs:/theme/icon_image.png
-icon_file=romfs:/theme/icon_file.png
-icon_folder=romfs:/theme/icon_folder.png
-icon_zip=romfs:/theme/icon_zip.png
-icon_nro=romfs:/theme/icon_nro.png

assets/romfs/themes/base_black_theme.ini

@@ -0,0 +1,15 @@
+[theme]
+background=0x2d2d2dff
+grid=0x46464630
+selected=0x464646ff
+selected_overlay=0x00ffc8ff
+text=0xfbfbfbff
+text_selected=0x00ffc8ff
+
+icon_audio=romfs:/theme/icon_audio.png
+icon_video=romfs:/theme/icon_video.png
+icon_image=romfs:/theme/icon_image.png
+icon_file=romfs:/theme/icon_file.png
+icon_folder=romfs:/theme/icon_folder.png
+icon_zip=romfs:/theme/icon_zip.png
+icon_nro=romfs:/theme/icon_nro.png

assets/romfs/themes/black_theme.ini

@@ -2,22 +2,4 @@
 name=Black
 author=TotalJustice
 version=1.0.0
-preview=romfs:/theme/preview.jpg
-
-[theme]
-background=0x2d2d2dff
-cursor=romfs:/theme/cursor.png
-cursor_drag=romfs:/theme/cursor_drag.png
-grid=0x46464630
-selected=0x464646ff
-selected_overlay=0x00ffc8ff
-text=0xfbfbfbff
-text_selected=0x00ffc8ff
-
-icon_audio=romfs:/theme/icon_audio.png
-icon_video=romfs:/theme/icon_video.png
-icon_image=romfs:/theme/icon_image.png
-icon_file=romfs:/theme/icon_file.png
-icon_folder=romfs:/theme/icon_folder.png
-icon_zip=romfs:/theme/icon_zip.png
-icon_nro=romfs:/theme/icon_nro.png
+inherit=romfs:/themes/base_black_theme.ini

assets/romfs/themes/oled_black_theme.ini

@@ -2,22 +2,12 @@
 name=OLED Black
 author=iTotalJustice/Sanras
 version=1.0.0
-preview=romfs:/theme/preview.jpg
+inherit=romfs:/themes/base_black_theme.ini
 
 [theme]
 background=0x000000ff
-cursor=romfs:/theme/cursor.png
-cursor_drag=romfs:/theme/cursor_drag.png
 grid=0x46464640
 selected=0x323232ff
 selected_overlay=0x00ffc8ff
 text=0xfbfbfbff
 text_selected=0x00ffc8ff
-
-icon_audio=romfs:/theme/icon_audio.png
-icon_video=romfs:/theme/icon_video.png
-icon_image=romfs:/theme/icon_image.png
-icon_file=romfs:/theme/icon_file.png
-icon_folder=romfs:/theme/icon_folder.png
-icon_zip=romfs:/theme/icon_zip.png
-icon_nro=romfs:/theme/icon_nro.png