Replace specified font-family on the body element.

replace-specified-font-family.js

const EXCLUSIONS = [
    'Meiryo',
    'メイリオ',
    '"MS PGothic"',
    '"ＭＳ ゴシック"',
    'x-locale-body',
    'system-ui',
    '"Yu Gothic UI"',
    '"YuGothic UI"'
].map(f => f.toLowerCase());

const ALT = '"游ゴシック Medium"';

const replaceFontFamily = () => {

    const css = (element, property) => window.getComputedStyle(element, null).getPropertyValue(property);

    const fonts = css(document.body, 'font-family').split(',').map(f => f.trim());

    if (fonts.length === 0) return;

    console.log('Replace Specifiied Font Family: original font family:', fonts);

    const newFonts = [...new Set(fonts.map(f => {
        if (EXCLUSIONS.includes(f.toLowerCase())) {
            console.log('Replace Specifiied Font Family: ', 'found: ', f);
            return ALT;
        }
        return f;
    }))];
    document.body.style.fontFamily = newFonts;
    console.log('Replace Specifiied Font Family: current font-family: ', newFonts);
};

window.addEventListener('load', replaceFontFamily, false);