Paste your signature's HTML below and see how it holds up when the inbox goes dark. Everything runs in your browser — nothing is uploaded, stored or sent anywhere. Built by ProSignature (free, works with any signature, not just ours).
Email clients disagree about what dark mode means. Gmail selectively recolors: light backgrounds go dark, dark text goes light — but only when it is confident, so hard-coded colors sometimes survive and vanish against the new background. Some Outlook builds invert everything, which turns a dark logo light. And Apple Mail mostly leaves HTML alone, so a signature designed only for white can sit in a glowing white box inside a dark inbox.
The five failures the checker looks for — near-black hard-coded text, transparent dark logos, oversized widths, sub-11px fonts and CSS background images — cover most real-world breakage we see. A signature built with the ProSignature generator passes these by construction, but this tester works on any signature from any tool.
No. The tester runs entirely in your browser. Your HTML never leaves the page — there is no server, no storage and no account.
Usually because it is a dark-colored image on a transparent background: on white it reads fine, on a dark inbox it blends into the background. The fix is a version of the logo with a solid background, or light padding baked into the image itself.
It is an approximation of Gmail's recolor behavior and of full-inversion clients. Clients differ by version and OS, so after fixing what the checks flag, send yourself a real test email in dark mode — that is the only ground truth.