Built in
Nine, under View → Theme (and in the ⌘K command bar as Theme: …):Paper: a launch checklist on light glass.
Following the system
Settings → Themes → Appearance. Always the chosen theme keeps whatever you picked. Follow the system takes one light theme and one dark theme — Paper and Dusk to begin with — and switches between them when macOS does. Choosing a theme from the View menu while following the system fills the matching slot.Making your own
Built-in themes cannot be edited, but any of them can be the starting point.1
Duplicate
Settings → Themes. Select the built-in theme closest to what you want and press +. A copy appears under Mine and becomes the current theme.
2
Edit, and watch
Everything you change is live in the main window behind the settings card — the tint, the grain, every colour — so you can judge it on your own text.
3
Keep or discard
− deletes a theme of yours. The ••• menu exports the current theme or imports one.
What you can change
Name, and whether it is a dark appearance (which decides the GitHub Review style’s colours and how the app treats it when following the system). Glass. The material, from a list:
Then the tint colour, tint strength (how much of the blur it covers, 0–100%), paper grain (a faint texture, 0–20%) and sidebar tint (how strongly the sidebar’s own darker tint is applied).
Colours. Text, Headings, Accent, Markdown syntax, Quotes, Code, Links, Caret and Selection. The accent is the most visible: it colours tags, date capsules, task boxes, the star, the selected sidebar row and, unless you set one, the caret.
Sharing a theme
Themes export as small JSON files,Name.glassinetheme.json, from the ••• menu in Settings → Themes. Import one from the same menu. They are easy to keep in a dotfiles repo, send to someone, or post; two of the built-ins, Dusk and Lavender, began life as exported files.