Replace the font with Sarasa Gothic (CJK) and the corresponding monospaced one.

Sarasa-Gothic.user.js

// ==UserScript==
// @name                 Sarasa-Gothic-Font-Substitution-Tool
// @name:en-US           Sarasa Gothic Font Substitution Tool
// @name:zh-CN           更纱黑体字体替换工具
// @name:zh-TW           更紗黑體字型替換工具
// @name:zh-HK           更紗黑體字型替換工具
// @name:ja-JP           更紗ゴシックフォント置換ツール
// @name:ko-KR           사라사고딕 폰트 대체 도구
// @namespace            https://gist.github.com/zozovo/8a76915acf197a873304dd23f2cfd49c
// @version              8.8
// @author               zozovo
// @description          Replace the font with Sarasa Gothic (CJK) and the corresponding monospaced one.
// @description:en-US    Replace the font with Sarasa Gothic (CJK) and the corresponding monospaced one.
// @description:zh-CN    将字体替换为更纱黑体（中日韩）和对应的等宽字体。
// @description:zh-TW    將字體替換為更紗黑體（中日韓）和對應的等寬字體。
// @description:zh-HK    將字體替換為更紗黑體（中日韓）和對應的等寬字體。
// @description:ja-JP    フォントを更紗ゴシック（中日韓）および対応する等幅フォントに置き換えます。
// @description:ko-KR    글꼴을사라사고딕(중일한)및해당등폭글꼴로대체합니다.
// @match                *://*/*
// @run-at               document-start
// @grant                GM_addStyle
// @license              MIT
// @supportURL           https://gist.github.com/zozovo/8a76915acf197a873304dd23f2cfd49c
// ==/UserScript==

const LANG_CODE_OBJ = {
    "zh-CN": "SC",
    "zh-SG": "SC",
    "zh-TW": "TC",
    "zh-HK": "HC",
    "ja": "J",
    "ko": "K",
    "ko-KR": "K",
    "ko-KP": "K",
};

const lang_code = LANG_CODE_OBJ[navigator.language] || "SC"; // Default to SC if not found

const FONT_FAMILY_TYPE = {
    generic: "var(--icon-font-family), var(--generic-font-family), system-ui;",
    code: "var(--icon-font-family), var(--code-font-family), monospace;",
};

const EMOJIS_UNICODE_RANGE = // Unicode CLDR: check https://unicode.org/emoji/charts/emoji-style.txt
    // "U+00A9, U+00AE" + // BMP: Latin-1 Supplement: copyright and registered
    "U+200C, U+200D, U+203C, U+2049, " + // BMP: General Punctuation: ZWNJ and ZWJ, double exclamation mark, exclamation question mark
    "U+20E3, " + // BMP: Combining Diacritical Marks for Symbols: combining enclosing keycap
    // "U+2122, " + // BMP: Letterlike Symbols: trade mark
    "U+2139, " + // BMP: Letterlike Symbols: information
    "U+2190-21FF, " + // BMP: Arrows
    "U+23??, " + // BMP: Miscellaneous Technical
    "U+24C2, " + // BMP: Enclosed Alphanumerics: circled M
    "U+25A0-27BF, " + // BMP: Geometric Shapes, Miscellaneous Symbols, Dingbats
    "U+2B??, " + // BMP: Miscellaneous Symbols and Arrows
    "U+2934, U+2935, " + // BMP: Supplemental Arrows-B: right arrow curving up, right arrow curving down
    "U+3030, U+303D, " + // BMP: CJK Symbols and Punctuation: wavy dash, part alternation mark
    "U+3297, U+3299, " + // BMP: Enclosed CJK Letters and Months: Japanese “congratulations” button, Japanese “secret” button
    "U+FE0E, U+FE0F, " + // BMP: Variation Selectors: VS15, VS16
    "U+1F???"; // SMP

GM_addStyle(`
@font-face {
    font-family: "Color Emoji";
    src: local("Apple Color Emoji"), local("Noto Color Emoji"), local("Segoe UI Emoji");
    unicode-range: ${EMOJIS_UNICODE_RANGE};
}

@font-face {
    font-family: "Monochrome Emoji";
    src: local("Apple Monochrome Emoji"), local("Noto Emoji"), local("Segoe Fluent Icons"), local("Segoe MDL2 Assets");
    unicode-range: ${EMOJIS_UNICODE_RANGE};
}

@font-face {
    font-family: "Consolas";
    src: local("Sarasa Fixed ${lang_code}")
}

@font-face {
    font-family: "Courier New";
    src: local("Sarasa Fixed ${lang_code}")
}

:root {
    --icon-font-family: "Color Emoji", "Monochrome Emoji";
    --generic-font-family: "Sarasa UI ${lang_code}";
    --code-font-family: "Sarasa Fixed ${lang_code}";
}

:is([lang$="TW"], [lang$="Hant"]) {
    --generic-font-family: "Sarasa UI TC";
    --code-font-family: "Sarasa Fixed TC";
}

:is([lang$="HK"], [lang$="MO"]) {
    --generic-font-family: "Sarasa UI HC";
    --code-font-family: "Sarasa Fixed HC";
}

:lang(ja) {
    --generic-font-family: "Sarasa UI J";
    --code-font-family: "Sarasa Fixed J";
}

:lang(ko) {
    --generic-font-family: "Sarasa UI K";
    --code-font-family: "Sarasa Fixed K";
}

:not(#_#_) {
    /* google-symbols https://fonts.google.com/icons,
     * font-awesome https://fontawesome.com/iconsother,
     * other icons
     */
    :not([class*="-symbols"], [class*="fa-"], [class*="icon"]) {
        /* generic fonts, except mathjax, <i> <s> for icons, <a> <span> for inherited */
        &:not(a, i, s, span, textarea, button *):not(
                math,
                math *,
                [aria-hidden="true"],
                [role="presentation"],
                [role="none"],
                [aria-hidden="true"] *,
                [role="presentation"] *,
                [role="none"] *
            ) {
            font-family: ${FONT_FAMILY_TYPE.generic}

            /* mono fonts */
            &:where(code, kbd, pre, samp, var),
            /* code editors: Ace https://ace.c9.io/,
             * CodeMirror https://codemirror.net/ https://codemirror.net/5/,
             * Monaco https://microsoft.github.io/monaco-editor/,
             * TailwindCSS https://tailwindcss.com/docs/font-family 
             */
            &:where([class*="ace_editor"], [class*="cm-"], [class*="monaco-editor"], [class*="font-mono"]) {
                font-family: ${FONT_FAMILY_TYPE.code}

                /* <i> for icon, <a> <span> for inherited */
                & :not(a, i, span) {
                    font-family: ${FONT_FAMILY_TYPE.code}
                }
            }

            /* for github readme*/
            &:where([class*="code"]:not(#readme, .readme)) {
                font-family: ${FONT_FAMILY_TYPE.code}
            }
        }
    }

    /* discuz */
    :is(a[class*="xst"]) {
        font-family: ${FONT_FAMILY_TYPE.generic}
    }
}
`);

@Leoyzen brilliant!