Thank you for reaching out and posting this question!
I understand you're looking to add a "Sign up" or "Log in " button to your homepage, and add a border around them. Since you're using a Shopify-supported theme, then this is something that our Theme Support team can explore for you. If your store is subscribed to a plan, you're eligible to use the complimentary design time included with your subscription on customizing your theme. While we’re not able to provide account-specific support via the Shopify Community at this time, we’d be happy to continue assisting you through live chat, email, or callback. Please visit the Shopify Help Center and log in to your account to create a support request.
The Venture theme does come with an "Account" button in the header, would changing the text to "Log in" or "Sign up" work as a solution?
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