override font-family-modern font weight live

src/main.js

@@ -52,6 +52,10 @@ const applyFontFamily = async function () {
     '--font-family-modern',
     fontFamily === 'custom' ? customFontFamily : fontFamily
   );
+  const { fontWeightOverride } = await import(browser.runtime.getURL('/override_font_weight.js'));
+  document.getElementById('palettes-for-tumblr-override')?.remove();
+  (fontFamily === 'custom' ? customFontFamily : fontFamily) &&
+    document.documentElement.append(fontWeightOverride);
 };
 
 const applyFontSize = async function () {

src/manifest.json

@@ -39,7 +39,7 @@
   },
   "permissions": [ "storage" ],
   "web_accessible_resources": [{
-    "resources": [ "*.json" ],
+    "resources": [ "*.json", "*.js" ],
     "matches": [ "*://www.tumblr.com/*" ]
   }],
 

src/override_font_weight.js

@@ -0,0 +1,49 @@
+await new Promise((resolve) =>
+  document.readyState === 'loading'
+    ? document.addEventListener('readystatechange', resolve, { once: true })
+    : resolve()
+);
+
+export const fontWeightOverride = Object.assign(document.createElement('style'), {
+  id: 'palettes-for-tumblr-override'
+});
+
+const getSelectorsFromUrl = async (url) => {
+  const cssText = await fetch(url).then(response => response.text());
+  const sheet = await new CSSStyleSheet().replace(cssText);
+
+  const allCssStyleRules = [...sheet.cssRules]
+    .filter((rule) => rule instanceof CSSStyleRule && rule.style)
+    .map(({ selectorText, style }) => ({
+      selectorText,
+      rules: Object.fromEntries([...style].map((key) => [key, style.getPropertyValue(key)]))
+    }));
+
+  return allCssStyleRules
+    .filter(
+      ({ rules }) =>
+        rules['font-family'] === 'var(--font-family-modern)' &&
+        rules['font-weight'] === '350'
+    )
+    .map(({ selectorText }) => selectorText);
+};
+
+const cache = {};
+
+const processLinkElements = async () => {
+  [...document.head.querySelectorAll('link[rel="stylesheet"][href^="https://assets.tumblr.com/pop/"]')]
+    .map(element => element.href)
+    .forEach(url => {
+      cache[url] ??= getSelectorsFromUrl(url);
+    });
+
+  const selectors = await Promise.all(Object.values(cache)).then(results => results.flat());
+  fontWeightOverride.textContent = `
+    ${selectors.join(', ')} {
+      font-weight: normal;
+    }
+  `;
+};
+
+processLinkElements();
+new MutationObserver(processLinkElements).observe(document.head, { childList: true });