How do I create yellow cards on a cork board background as a default?
The options you want are all in Preferences > Appearance.
For the card background colour, choose Index Cards > Colours > Index Card Background
For the corkboard itself, choose Corkboard > Colours > Corkboard Background.
However, if you want to color code your index cards using the colors of Labels, instead of just yellow, you can change the index cards to use any set of colors that have a specific meaning (PoV character, scene setting, timeline…). Whatever you want to put the label feature to use for. You can turn that setting on via the View->Use Label Color In… menu.