Metre2 Mail - free
Metre2 Mail 1.2 has a calm new look. Light mode is blueprint paper with ink-blue lines; dark mode is deep navy with cyan. Folders are numbered, times and labels are set in a clear mono type, and every email is shown on its own sheet of clean white paper - sandboxed, with a caption that says what was blocked.

Sheet 01DarkDeep navy
Click the cog at the bottom of the left rail and open the Reading tab.
Look: Match Windows follows your Windows light or dark setting, or choose Light or Dark to keep one.
Whichever you choose, each email sits on clean white paper so it looks as the sender meant - with a mono caption under it: sandboxed, remote images blocked, links cleaned.
With no email open, a blueprint drawing of an envelope fills the reading pane with how many are unread. It's drawn once and then sits still, so it costs nothing while Mail is idle.

Help: Settings and the look has every option in detail.
Free in Metre2 Mail. No time limit, no ads, no account, and your mail never passes through our servers. Mail Pro is there when you want every account in one inbox, categories, AI drafts and unlimited rules.
There are two looks, light and dark, plus Match Windows - that's deliberate, to keep it calm and simple.
No. Each email stays on white paper, so it looks the way its sender designed it, and pictures and signatures don't go wrong.
No. It's drawn once when the window opens or changes size, then sits still - no animation and no work while Mail is idle. It hides on narrow windows.
Yes, for everyone.

Work and school mail through Microsoft Graph - even where IMAP is switched off.

PST and OST files folder by folder, .mbox and .eml too - duplicates skipped.

Summaries free; reply drafts and translations with Pro - nothing leaves your PC.

Three free rules, and one right-click to make one. Unlimited with Pro.

Pin to the top with P, free. Colour-code with C in Mail Pro.

Tracking pictures hidden, links cleaned, every email sandboxed, fakes flagged.
Metre2 Mail is free. Mail Pro is a one-off £9.99, with a free 7-day trial.