Skip to main content
Chainlit’s theme is based on CSS variables. To modify the CSS variables, create a theme.json file under /public with the following content. You can check Shadcn’s documentation to learn about the role of each variable.
If the UI is not updated, try to empty your browser cache.
theme.json
{
    "custom_fonts": [],
    "variables": {
        "light": {
            "--font-sans": "'Inter', sans-serif",
            "--font-mono": "source-code-pro, Menlo, Monaco, Consolas, 'Courier New', monospace",
            "--background": "0 0% 100%",
            "--foreground": "0 0% 5%",
            "--card": "0 0% 100%",
            "--card-foreground": "0 0% 5%",
            "--popover": "0 0% 100%",
            "--popover-foreground": "0 0% 5%",
            "--primary": "340 92% 52%",
            "--primary-foreground": "0 0% 100%",
            "--secondary": "210 40% 96.1%",
            "--secondary-foreground": "222.2 47.4% 11.2%",
            "--muted": "0 0% 90%",
            "--muted-foreground": "0 0% 36%",
            "--accent": "0 0% 95%",
            "--accent-foreground": "222.2 47.4% 11.2%",
            "--destructive": "0 84.2% 60.2%",
            "--destructive-foreground": "210 40% 98%",
            "--border": "0 0% 90%",
            "--input": "0 0% 90%",
            "--ring": "340 92% 52%",
            "--radius": "0.75rem",
            "--sidebar-background": "0 0% 98%",
            "--sidebar-foreground": "240 5.3% 26.1%",
            "--sidebar-primary": "240 5.9% 10%",
            "--sidebar-primary-foreground": "0 0% 98%",
            "--sidebar-accent": "240 4.8% 95.9%",
            "--sidebar-accent-foreground": "240 5.9% 10%",
            "--sidebar-border": "220 13% 91%",
            "--sidebar-ring": "217.2 91.2% 59.8%"
        },
        "dark": {
            "--font-sans": "'Inter', sans-serif",
            "--font-mono": "source-code-pro, Menlo, Monaco, Consolas, 'Courier New', monospace",
            "--background": "0 0% 13%",
            "--foreground": "0 0% 93%",
            "--card": "0 0% 18%",
            "--card-foreground": "210 40% 98%",
            "--popover": "0 0% 18%",
            "--popover-foreground": "210 40% 98%",
            "--primary": "340 92% 52%",
            "--primary-foreground": "0 0% 100%",
            "--secondary": "0 0% 19%",
            "--secondary-foreground": "210 40% 98%",
            "--muted": "0 1% 26%",
            "--muted-foreground": "0 0% 71%",
            "--accent": "0 0% 26%",
            "--accent-foreground": "210 40% 98%",
            "--destructive": "0 62.8% 30.6%",
            "--destructive-foreground": "210 40% 98%",
            "--border": "0 1% 26%",
            "--input": "0 1% 26%",
            "--ring": "340 92% 52%",
            "--sidebar-background": "0 0% 9%",
            "--sidebar-foreground": "240 4.8% 95.9%",
            "--sidebar-primary": "224.3 76.3% 48%",
            "--sidebar-primary-foreground": "0 0% 100%",
            "--sidebar-accent": "0 0% 13%",
            "--sidebar-accent-foreground": "240 4.8% 95.9%",
            "--sidebar-border": "240 3.7% 15.9%",
            "--sidebar-ring": "217.2 91.2% 59.8%"
        }
    }
}
As you may have noticed, the colors are not expressed in Hexadecimal but rather in HSL. This is mandatory. You can easily convert any color to HSL. The custom_fonts array can receive URLs (typically from google fonts) like: 
custom_fonts: ["https://fonts.googleapis.com/css2?family=Inter:wght@400;500;700&display=swap"]