To improve website speed, I want to remove google font and use system font to instead:

First, I need to remove <link href='https://fonts.googleapis.com/css?family=Open+Sans' rel='stylesheet' type='text/css'>

There are font-family in 25 positions in stye.css
How and where to insert following system fonts into css?

[brown]font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;[/brown]

Is there any other thing I have to do?

Hi

In style.css you would replace each instance of

font-family:Open Sans, sans-serif;

With what you want to use so from your example and an example from style.css

body{padding:0;margin:0;font-family:Open Sans, sans-serif;font-size:18px;color:#000}

would become

body{padding:0;margin:0;font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Oxygen-Sans, Ubuntu, Cantarell, "Helvetica Neue", sans-serif;font-size:18px;color:#000}

Andy

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Hi Andy, thanks.

The font seems work well, however I found that the layout changed.
Left sidebox and body content not in the center.
How to fix it?

Is the only change you made related to the font-family, nothing else?

Andy

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Andy, thank for your reply.

I have solved the issue.