Under your shopify dashboard, navigate to "themes" under "online stores" which is under "Sales channels". Here, click on customize button for the current theme you are using. Navigate to the page, where you want to change the story border color.

Go to the PDP where you want to change the video carousel style, click on the carousel section.

Under the customization sidebar, there is a drop down called "Tile Type", click on it, and change it to your desired style.