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 whatmore video shopping section and click on the app.

Custom CSS section opens, click on the downward arrow, and you can add the CSS in the text field.

CSS is implemented and, here the color of the font is changed to red, similarly you can customize the video elements using other CSS code.