style: modify font-synthesis from none to style

[Timeless0911]

Summary

change the global styles of font-synthesis from none to style

Consistent with Vitepress: https://github.com/vuejs/vitepress/blob/7ed5148260052840c44bae64a670d8d610999dbe/src/client/theme-default/styles/base.css#L42

Related Issue

closes: #984

Checklist

• [ ] Tests updated (or not required).
• [ ] Documentation updated (or not required).

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