I understand that you want to add border in your buble cart icon.
Try this one.
From your Shopify admin dashboard, click on "Online Store" and then "Themes".
Find the theme that you want to edit and click on "Actions" and then "Edit code".
In the "Assets" folder, click on "theme.scss.liquid, styles.scss.liquid or base.css" file, depending on which file your theme uses to store its CSS styles. At the bottom of the file, add the following CSS code:
border: .5px solid black;
You can adjust anytime the size.
I hope it help.
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to the post that helped you. Thanks! Sip, savor, and support ☕ – your