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