[material-ui][pigment-css] Support project without enabling CSS variables #44171
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closes #43750
Root cause
The
.toRuntimeSource
is required in the theme for some components that access runtime theme's transitions (e.g. Slide) but the property only exist when using CSS theme variables.Pigment CSS should work with any theme, so the fix is to also add the property to the theme when CSS variables is not enabled.