fix(css): correct disunited font size in code block

[Unbinilium]

I notified the font size of code blocks is abnormal on my iOS devices (only small screen devices, debugging remotely is fine), assuming that would be a WebKit bug firstly but there hasn't any fixes these years. Considering that would happen on other browsers whose devices has a small screen, disable unwanted auto-enlargement by uniting text-size-adjust percentage of small texts would correct the font size in code blocks.

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