fix(go.dev): adjust dark/light application selectors

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🔧 What does this fix? 🔧

Sometimes the data-theme attribute doesn't exist, like if it is your first time visiting the website. This adjusts the theme application selectors to use prefers-color-scheme if there is no attribute set, as if data-theme="auto".

🗒 Checklist 🗒

• I have read and followed Catppuccin's contributing guidelines.
• I have updated the version appropriately in the ==UserStyle== header of the catppuccin.user.css file.

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FYI for userstyles you maintain, you have the responsibility of merging PRs once you've approved them (see https://github.com/catppuccin/userstyles/blob/main/docs/CONTRIBUTING.md#pull-request-review-process) :)