Sweep: Refactor ColorModeSwitcher to class component.

[sweep-nightly]

• In src/ColorModeSwitcher.tsx, define a class ColorModeSwitcher that extends React.Component.
• In src/ColorModeSwitcher.tsx, implement a render method that returns the same JSX as the functional component.
• In src/ColorModeSwitcher.tsx, replace the use of useColorMode and useColorModeValue with this.state and this.setState.
• In src/ColorModeSwitcher.tsx, write unit tests for the refactored component and run them to verify that they pass.

Parent issue: #335

[sweep-nightly]

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