LearnDash already provides built-in templates and also the color types to make the look and feel of the website more attractive.
To Upgrade the design just follow these steps:
go to the main dashboard of the WordPress >> select the LearnDash option then go to the settings >> there below you will find the design panel with every Possible Design option.
If you want to add more templates you can go for the AddOn on the main site. LearnDash.Com