Added version note for automatic theme

Relates to #341

_locales/de/messages.json

@@ -1032,7 +1032,7 @@
 		"description": ""
 	},
 	"theme_description": {
-		"message": "",
+		"message": "Das automatische Theme schaltet auf dunkel, wenn das System auch ein dunkles Theme verwendet (benötigt Firefox >= 67).",
 		"description": ""
 	},
 	"theme_options.auto": {

_locales/en/messages.json

@@ -1079,7 +1079,7 @@
 		"description": ""
 	},
 	"theme_description": {
-		"message": "",
+		"message": "The automatic theme switches to dark if the system uses a dark theme (requires Firefox >= 67)",
 		"description": ""
 	},
 	"theme_options.auto": {

releaseNotes.txt

@@ -11,7 +11,7 @@ Version 0.5.9:
 		- changed input of lists to textarea
 		- added option to protect no part of the canvas API
 		- apply themes to all extension pages (options, page action, browser action, setting sanitation, setting inspection, navigator settings)
-		- theme for automatic detection of dark mode
+		- theme for automatic detection of dark mode (only works with Firefox >= 67)
 	
 	fixes:
 		- search could show hidden settings