A self-service portal is a feature in Service Cloud that allows creating branded websites and portals for customers to access self-service resources, such as knowledge articles, case management, chat, etc. To ensure that the theme of the self-service portal is consistent with the theme of the B2C Commerce storefront, a Solution Architect should do the following:
Set the value of the Style Sheet URL setting to match the publicly-accessible URL of the style sheet the organization wants to use. The Style Sheet URL setting is a configuration option in Service Cloud that allows specifying a custom style sheet for the self-service portal. The style sheet must be hosted on a publicly-accessible web server and must follow certain guidelines and limitations. By setting the value of the Style Sheet URL setting to match the style sheet used by the B2C Commerce storefront, the Solution Architect can ensure that the self-service portal has the same look and feel as the storefront.
Option B is incorrect because there is no Portal Theme URL setting in Service Cloud. Option C is incorrect because copying any relevant CSS code from the organization's website and pasting it into the pages for the self-service portal in Page Builder is not a recommended or supported way of customizing the theme of the self-service portal. Option D is incorrect because making a copy of the appropriate CSS file from the organization's web server and uploading it to the self-service portal is not possible or advisable. The CSS file must be hosted on a publicly-accessible web server and referenced by the Style Sheet URL setting. Reference:
https://help.salesforce.com/s/articleView?id=sf.networks_overview.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.selfservice_style_sheet.htm&type=5

This answer is correct because it aligns with the executive team's request to see the benefits and return on investment sooner than three years. A multi-phased approach allows for incremental delivery of functionality across all clouds, which can provide value to the business and customers faster and more frequently. Reference: https://trailhead.salesforce.com/en/content/learn/modules/solution-design/solution-design-process