The Mermaid Themes plugin allows Obsidian users to customize and apply themes to their mermaid.js diagrams. Users can either provide a complete mermaid theme object or modify specific parts of the base theme, with the plugin merging changes seamlessly. This enhances the visual appeal of diagrams by supporting advanced theming options. Users are required to use an alternative code block label (merm) instead of the default mermaid to integrate themes. The plugin leverages mermaid.js theming features, providing a flexible way to improve diagram aesthetics in notes.