[v5-next] MuiSelect styleOverrides not being rendered

The styles applied in the stylesOverrides are not being shown on the Select components.

• The issue is present in the latest release.
• I have searched the issues of this repository and believe that this is not a duplicate.

Current Behavior 😯

The component renders without custom styling applied.

Expected Behavior 🤔

The component should have custom styling applied from the theme.

Steps to Reproduce 🕹

CodeSandbox.io

Check out the App.js file; within the theme you should see defaultProps and overrideStyles written but only defaultProps being applied.

Context 🔦

Trying to apply the styles globally to remove code repitition.

Your Environment 🌎

`npx @material-ui/envinfo`

  System:
    OS: macOS 12.0
    CPU: (8) x64 Intel(R) Core(TM) i5-8257U CPU @ 1.40GHz
    Memory: 41.63 MB / 16.00 GB
    Shell: 5.8 - /bin/zsh
  Binaries:
    Node: 14.16.0 - ~/.nvm/versions/node/v14.16.0/bin/node
    Yarn: 1.22.10 - /usr/local/bin/yarn
    npm: 7.11.0 - ~/.nvm/versions/node/v14.16.0/bin/npm
  Managers:
    CocoaPods: 1.10.1 - /usr/local/bin/pod
    Homebrew: 3.1.2 - /usr/local/bin/brew
    pip3: 20.2.3 - /usr/local/bin/pip3
    RubyGems: 3.0.3 - /usr/bin/gem
  Utilities:
    Make: 3.81 - /usr/bin/make
    GCC: 4.2.1 - /usr/bin/gcc
    Git: 2.23.0 - /usr/local/bin/git
    Clang: 12.0.5 - /usr/bin/clang
  Servers:
    Apache: 2.4.48 - /usr/sbin/apachectl
  SDKs:
    iOS SDK:
      Platforms: iOS 14.5, DriverKit 20.4, macOS 11.3, tvOS 14.5, watchOS 7.4
  IDEs:
    Android Studio: 3.6 AI-192.7142.36.36.6392135
    Nano: 2.0.6 - /usr/bin/nano
    Vim: 8.2 - /usr/bin/vim
    Xcode: 12.5.1/12E507 - /usr/bin/xcodebuild
  Languages:
    Bash: 3.2.57 - /bin/bash
    Java: 13.0.1 - /usr/bin/javac
    Perl: 5.30.3 - /usr/bin/perl
    Protoc: 3.13.0 - /usr/local/bin/protoc
    Python: 2.7.16 - /usr/bin/python
    Python3: 3.8.6 - /usr/local/bin/python3
    Ruby: 2.6.3 - /usr/bin/ruby
  Databases:
    MySQL: 10.15 - /usr/local/bin/mysql
    SQLite: 3.35.3 - /usr/bin/sqlite3
  Browsers:
    Chrome: 90.0.4430.212
    Edge: 88.0.705.74
    Firefox: 87.0
    Safari: 15.0

[aidanm1999]

Thanks for all your help @mnajdova. For reference, I ended up with the following code in the theme which produced the outlook I was aiming to achieve:

MuiSelect: {
    defaultProps: {
        variant: "outlined",
        size: "small",
        native: true,
        IconComponent: KeyboardArrowDownRounded,
    },
},
MuiOutlinedInput: {
    styleOverrides: {
        root: {
            fontSize: "0.9rem !important",
            fontFamily: Font.SemiBold,
            borderRadius: 100,
            padding: 0,
            borderColor: Colors.LightGrey,
        },
    },
},
MuiNativeSelect: {
    styleOverrides: {
        select: {
            padding: "6px 28px 4px 12px !important",
        },
    },
},

[mnajdova]

Looks the changes are applied in the migration-v4.md guide, I will be closing this one. @aidanm1999 feel free to re-open if there is anything else we may have missed.