[material-ui] Keep CSS theme variables out of the default theme

[Janpot]

defaultTheme is built through the public createTheme(), which statically imports both createThemeNoVars and createThemeWithVars and selects one at runtime based on the cssVariables option. Because that dispatch is a runtime branch, bundlers can't tree-shake the unused CSS-variables path, so the whole createThemeWithVars machinery (and its helpers: createColorScheme, createGetSelector, excludeVariablesFromRoot, getOverlayAlpha, prepareTypographyVars) ends up in every component bundle that only needs the default fallback theme.

createTheme() with no arguments is definitionally identical to createThemeNoVars() — the default cssVariables: false path returns createThemeNoVars(options), and createThemeNoVars does not depend on createThemeWithVars. Importing it directly produces a byte-identical default theme while letting the dead CSS-vars code be dropped.

The public createTheme export is untouched, so consumers who opt into CSS theme variables are unaffected.

Bundle size impact

Measured with test/bundle-size (gzip):

bundle	before	after	change
@mui/material/Button	25.29 kB	20.55 kB	-18.8%
@mui/material/Typography	18.76 kB	14.07 kB	-25.0%
@mui/material/IconButton	24.27 kB	19.54 kB	-19.5%
@mui/material/Checkbox	25.53 kB	20.76 kB	-18.7%
@mui/material/Slider	25.33 kB	20.57 kB	-18.8%
@mui/material/Switch	25.20 kB	20.45 kB	-18.8%
@mui/material (barrel)	148.92 kB	148.92 kB	unchanged
@mui/material/styles#createTheme	14.57 kB	14.57 kB	unchanged

~4.7 kB gzip (~18 kB parsed) off essentially every component, while the @mui/material barrel and the public createTheme entry are unchanged.

[code-infra-dashboard]

Deploy preview

https://deploy-preview-48607--material-ui.netlify.app/

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	▼-1B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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