[FR]: Fix theming.md dim-level examples to show composable context

Describe the problem

The "Dim levels" section in theming.md shows code like:

val disabledColour = SparkTheme.colors.onSurface.disabled

Both .disabled and .dim2 are @Composable getters. The code shown has no @Composable context and will not compile.

Describe the solution

Wrap the examples in a @Composable function or show the properties used inside a composable body.

Additional context

Priority: Medium. Effort: XS.