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The glass shows whatever is behind the window. With the right wallpaper that is the whole point of it: colour and light moving softly behind the page. With a plain desktop, or a picture that fights the text, it is not much of anything. A backdrop puts the colour inside the window instead — long folds of it, like silk lit from one side, each with a lit side, a shadowed side and a thin bright edge where it turns, drifting slowly — so the glass has something to look through wherever it is. A backdrop is a set of three to five colours: ten come built in, and any of them can be copied and recoloured into one of your own. Settings → Themes → Behind the glass (the switch at the top of the Themes pane; Settings reopens on whichever half you left it on), or View → Backdrop, or Backdrop: … in the ⌘K command bar.

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The backdrops

Every backdrop takes its lightness from the theme, and is built dark first: on a dark theme the ground is close to black and the colour lives in the folds, deep rather than bright, so pale text stays readable over every part of it; a light theme gets the same folds as a pale morning. The theme’s tint and grain still lie over it, and the sidebar is a veil over it.

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Your own colours

Select a backdrop and press + under the list: a copy appears under Mine, selected, with its colours in wells on the right. Change any of them, remove one (down to three) or add one (up to five), and rename it; the window follows every change. Every colour in the set is on screen at once: the folds run round the set in the order you give, so each colour has folds of its own, and they trade places slowly as the backdrop drifts. The hue and saturation of each colour are what count — the theme sets the lightness, so a colour you pick as bright cobalt becomes deep cobalt on a dark theme and pale cobalt on a light one, and the text stays readable either way. The first colour also tints the ground. − deletes one of yours; ⌘Z takes back the last change, like any setting. Your backdrops are saved with the app’s settings, beside your custom themes. The folds are drawn by a small program on the graphics processor — a field of noise warped through itself, read as bands and lit — at a quarter of the window’s size, since there is nothing sharp in them, so they cost the writing nothing.

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Drift, frost and grain

Drift moves the folds, slowly — the picture a few minutes from now is a different one, but no moment of it moves enough to catch the eye. Twenty frames a second while the window can be seen, none while it is covered. Off under Reduce Motion, and off with the switch: a still picture then. Frost pales and softens the colour, the way frosted glass does — gently at first, since a little haze goes a long way over a dark ground, and all the way at 100%: at 0 the folds are as deep as they get; at 30%, the default, they are barely touched; at 100% they are a soft tint under a veil of light, their edges gone. Paper grain lies over the folds the way a theme’s grain lies over the desktop’s blur — a fine, still texture that keeps the colour from looking like a render. It is a setting of its own here rather than the theme’s, since silk wants more of it than glass: 8% is the default, against the themes’ 4 or 5. Screenshots taken by the app of itself show the backdrop too, since it is inside the window; that is how the pictures on this page were made.