Responsive and dark styles

Author responsive and dark email styles while keeping baseline content usable and reviewing compatibility findings for target clients.

Import CSS into the email project. Compilation applies the CSS cascade, inlines applicable rules and retains supported head and media rules. Tailwind uses its compiler and the project's styling configuration. Classes must be discoverable from source; dynamic class construction can produce a diagnostic instead of the intended styling.

Keep base content and layout usable without media rules. Pair foreground and background changes for dark styling. Inspect declared fixtures in light and dark preview, including long and empty content. Compatibility diagnostics describe known constraints; a browser preview does not emulate every mail client. Use actual inbox tests for client-specific appearance evidence.