Fix: useTheme check by removing default context value

[ursulabauer]

What changed? Why?
Removed the default value from ThemeContext (theme: 'dark', toggleTheme: () => {}) because it made useContext(ThemeContext) always return an object, making the useTheme check useless. Now, ThemeContext is initialized with null, allowing the check to work properly when ThemeProvider is missing.

Notes to reviewers
This ensures useTheme throws an error when used outside ThemeProvider, as originally intended. Let me know if you see any edge cases I might have missed.

How has it been tested?
Manually tested by rendering a component using useTheme inside and outside ThemeProvider to confirm the expected behavior.

Have you tested the following pages?

BaseWeb

• base.org
• base.org/names
• base.org/builders
• base.org/ecosystem
• base.org/name/jesse
• base.org/manage-names
• base.org/resources

BaseDocs

• docs.base.org
• docs sub-pages

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[cb-heimdall]

🟡 Heimdall Review Status

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Reviews	🟡 0/1	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 0 Sum 1