extendTheme utility included into final bundle, potential bundle size optimization

Duplicates

• I have searched the existing issues

Latest version

• I have tested the latest version

Steps to reproduce 🕹

Just installing the latest @mui/joy into the project. Analysing bundle size i noticed that extendTheme utility (~12kb size) is included into the production bundle. I am using default next.js with official create-next-app.

I was wondering if this is necessary? My understanding is that extendTheme utility is used to merge default and custom theme but if no custom theme is used I do not see why it would be included.

Current behavior 😯

extendTheme utility (~12kb size) is included with final bundle even though it is not imported anywhere.

Expected behavior 🤔

extendTheme utility (~12kb size) is NOT included with final bundle if it is not imported inside the project. If it is imported for custom theme support then it is of course logical that it is included.

Context 🔦

Just trying to optimize bundle size because ~12kb is 15% of the whole size of @mui/joy that is imported into my project/case.

Your environment 🌎

  System:
    OS: macOS 12.6
  Binaries:
    Node: 16.17.1 - ~/.volta/tools/image/node/16.17.1/bin/node
    Yarn: 1.22.18 - ~/.volta/tools/image/yarn/1.22.18/bin/yarn
    npm: 8.15.0 - ~/.volta/tools/image/node/16.17.1/bin/npm
  Browsers:
    Chrome: 106.0.5249.119
    Edge: Not Found
    Firefox: Not Found
    Safari: 16.0
  npmPackages:
    @emotion/react: ^11.10.4 => 11.10.4
    @emotion/styled: ^11.10.4 => 11.10.4
    @mui/base:  5.0.0-alpha.101
    @mui/core-downloads-tracker:  5.10.9
    @mui/icons-material: ^5.10.9 => 5.10.9
    @mui/joy: ^5.0.0-alpha.49 => 5.0.0-alpha.49
    @mui/material:  5.10.9
    @mui/private-theming:  5.10.9
    @mui/styled-engine:  5.10.8
    @mui/system:  5.10.9
    @mui/types:  7.2.0
    @mui/utils:  5.10.9
    @types/react: 18.0.21 => 18.0.21
    react: 18.2.0 => 18.2.0
    react-dom: 18.2.0 => 18.2.0
    typescript: 4.8.4 => 4.8.4