[IconButton] Refactor styles to use CSS variables

[BENABDERRAHMANE-GE]

This PR refactors IconButton styles to use component-level CSS variables.

• Moves padding, font-size and color values to CSS variables
• Preserves existing behavior and theme integration
• No API or behavior changes

Part of the Material → Material You migration.

[mui-bot]

Netlify deploy preview

https://deploy-preview-47572--material-ui.netlify.app/

Bundle size report

Bundle	Parsed size	Gzip size
@mui/material	🔺+286B(+0.06%)	🔺+65B(+0.04%)
@mui/lab	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

Generated by 🚫 dangerJS against da1dfc2

[ZeeshanTamboli]

Part of the Material → Material You migration.

We are not planning to migrate to Material You.

[BENABDERRAHMANE-GE]

@ZeeshanTamboli, Thanks for the review.

It was part of this issue :
#39943

This PR specifically addresses the last migration step listed in the issue (“Refactor styles to use component CSS Variables”, see #39943), by moving IconButton styles (padding, font-size, color) to component-level CSS variables while preserving existing behavior and theme integration.

Given that this step is now covered here, could you please confirm whether this part of the migration is still considered pending, or if the issue should be updated to reflect that this requirement is now addressed for IconButton?