Card Collection background color changes to medium brown (Wild format) after playing a match of Dalaran Heist. Screenshot below.
Have not tested if playing any other Adventure does the same thing.
Selecting a different set, changing page or Class restores the light tan (Standard format) background color.
System: Windows 10
https://imgur.com/a/c9dkx5D
Thanks for the report Fuschia! I suspect a Wild card in the adventure deck might be the cause. Let us know if that is not the case 
Further testing finds that one does not need to go as far as playing a match. Just entering the Adventure (Dalaran Heist, Rastakhan’s Rumble, The Witchwood, or Kobolds & Catacombs) with an active deck (run) causes the background change.
But doing a Wild format Class Challenge (The League of Explorers - Warrior) does not cause the background to change.
Edit: Playing a Practice game against the Innkeeper (with Wild deck) does not cause the background to change.
Edit2: Start a Dungeon Run (Kobolds & Catacombs) with Hunter and background still change. Deck has only Standard cards (list below incase overlooked something). This was starting deck before choosing a Treasure.
Stonetusk Boar
Dire Wolf Alpha
Explosive Trap
Hunter’s Mark
Animal Companion
Deadly Shot
Eaglehorn Bow
Jungle Panther
Unleash the Hounds
Oasis Snapjaw
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