fix(v2): set stored theme only if it exists

[lex111]

Motivation

Currently, automatic switching to a dark theme does not work if a dark theme is selected in the system (because even when there is no saved theme in local storage, we set (override init value with correct theme) the theme to null value which is cast to an empty string (i.e. light theme)). This PR solves this bug.

docusaurus/packages/docusaurus-theme-classic/src/index.js

Lines 29 to 30 in ba7c38c

	} else if (supportsColorSchemeQuery && mql.matches) {
	setDataThemeAttribute('dark');

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

1. Select a dark theme in your OS
2. Go to the docs site (after clearing localstorage), a dark theme should be enabled.

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[endiliey]

Shouldnt it be

let localStorageTheme = null;
try

catch

if locaStorageTheme !== null,
setTheme

[lex111]

I don't think so, the current code is simple and working.

[yangshun]

Thanks, I noticed this bug and saw that the page turned dark then turned back to light so I suspected it was this took overriding it after the inline script set it. Great job @lex111!

This is the correct fix @endiliey

[endiliey]

Yeah this is correct because getItem will always return null if key does not exist (local storage cleared). Always thought it was undefined.

Although i believe this below still achieve the same.

let localStorageTheme = null;
try {
localStorageTheme = localStorage.getItem(‘theme’)
}
catch {}
if (localStorageTheme !== null) {
setTheme(localStorageTheme)
}

[yangshun]

@endiliey where are you suggesting to add that?

I don't think the above code will work if you meant to put it in the fouc script.

[endiliey]

^ in the same code of this PR

Its just the same. But more code

Just forget it lol. 🤣