It's 20%, not 10%, per themed set.

(2) Vox Populi/Database Changes/Text/en_US/Units/MissionTextChanges.sql

@@ -15,7 +15,7 @@ WHERE Tag = 'TXT_KEY_MISSION_GIVE_POLICIES_HELP';
 
 -- Start a Golden Age
 UPDATE Language_en_US
-SET Text = '+{1_Num} [ICON_GOLDEN_AGE] Golden Age Points[NEWLINE][NEWLINE]This order will consume the Great Artist and create Golden Age Points, which may trigger a [ICON_GOLDEN_AGE] Golden Age (extra [ICON_PRODUCTION] Production, [ICON_GOLD] Gold, and [ICON_CULTURE] Culture). Scales based on the [ICON_TOURISM] Tourism and [ICON_GOLDEN_AGE] Golden Age Point output of the past 10 turns, and is increased by 10% for every owned [COLOR_POSITIVE_TEXT]themed[ENDCOLOR] Great Work set.'
+SET Text = '+{1_Num} [ICON_GOLDEN_AGE] Golden Age Points[NEWLINE][NEWLINE]This order will consume the Great Artist and create Golden Age Points, which may trigger a [ICON_GOLDEN_AGE] Golden Age (extra [ICON_PRODUCTION] Production, [ICON_GOLD] Gold, and [ICON_CULTURE] Culture). Scales based on the [ICON_TOURISM] Tourism and [ICON_GOLDEN_AGE] Golden Age Point output of the past 10 turns, and is increased by 20% for every owned [COLOR_POSITIVE_TEXT]themed[ENDCOLOR] Great Work set.'
 WHERE Tag = 'TXT_KEY_MISSION_START_GOLDENAGE_HELP';
 
 -- Perform Concert Tour