Refactor Theme Provider to not Mount MUI and CSS by Default

[jenny-s51]

Follow up to #16

The ThemeProvider automatically imports MUI styles at module level, causing:

• JavaScript injection into SCSS files during build (with sass as peer dependency)
• Prevents tree shaking - MUI styles bundled even when unused
• Stylesheet conflicts - unintended theme overrides in consuming apps

Solution: Refactor theme provider to not Mount MUI and CSS by default.

[lucferbux]

For posterity, we are gonna enforce some configurations in modular architecture so we'll make this a lower priority.

[lucferbux]

We can close this issue as we've covered this request in #26