diff --git a/lib/shared/theme.js b/lib/shared/theme.js
index ae14422e..79ea115d 100644
--- a/lib/shared/theme.js
+++ b/lib/shared/theme.js
@@ -100,13 +100,10 @@ export class ThemeManagerBase extends GObject.Object {
    * @param {any} selector
    */
   getCssRule(selector) {
-    if (this.cssAst) {
-      const rules = this.cssAst.stylesheet.rules;
-      // return only the first match, Forge CSS authors should make sure class names are unique :)
-      const matchRules = rules.filter((r) => r.selectors.filter((s) => s === selector).length > 0);
-      return matchRules.length > 0 ? matchRules[0] : {};
-    }
-    return {};
+    const rules = this.cssAst?.stylesheet?.rules ?? [];
+    // return only the first match, Forge CSS authors should make sure class names are unique :)
+    // (comments and @-rules have no selectors)
+    return rules.find((r) => r.selectors?.includes(selector)) ?? null;
   }
 
   /**
@@ -115,13 +112,7 @@ export class ThemeManagerBase extends GObject.Object {
    */
   getCssProperty(selector, propertyName) {
     const cssRule = this.getCssRule(selector);
-
-    if (cssRule) {
-      const matchDeclarations = cssRule.declarations.filter((d) => d.property === propertyName);
-      return matchDeclarations.length > 0 ? matchDeclarations[0] : {};
-    }
-
-    return {};
+    return cssRule?.declarations?.find((d) => d.property === propertyName) ?? {};
   }
 
   /**
@@ -143,16 +134,86 @@ export class ThemeManagerBase extends GObject.Object {
    * Returns the AST for stylesheet.css
    */
   _importCss() {
+    const defaultFile = this.configMgr.defaultStylesheetFile;
     let cssFile = this.configMgr.stylesheetFile;
     if (!cssFile || !production) {
-      cssFile = this.configMgr.defaultStylesheetFile;
+      cssFile = defaultFile;
     }
 
-    let [success, contents] = cssFile.load_contents(null);
-    if (success) {
-      const cssContents = new TextDecoder().decode(contents);
-      this.cssAst = parse(cssContents);
+    this.cssAst = cssFile ? this._parseCssFile(cssFile) : null;
+    if (!defaultFile || cssFile?.equal(defaultFile)) return;
+
+    // The user's stylesheet can be empty, from an older Forge or edited by hand: take the rules
+    // and properties Forge reads that it lacks from the default stylesheet, so that reading them
+    // doesn't fail and the shell styles everything (#448). If it can't be parsed at all, use the
+    // default stylesheet. Either way the stylesheet is saved completed, and the user's own file
+    // is kept as stylesheet.css.bak.
+    const defaultAst = this._parseCssFile(defaultFile);
+    if (!defaultAst) return;
+    let completed = false;
+    if (!this.cssAst) {
+      this.cssAst = defaultAst;
+      completed = true;
+    } else {
+      const rules = this.cssAst.stylesheet.rules;
+      for (const defaultRule of defaultAst.stylesheet.rules) {
+        if (!defaultRule.selectors) continue;
+        const rule = this.getCssRule(defaultRule.selectors[0]);
+        if (!rule) {
+          rules.push(defaultRule);
+          completed = true;
+          continue;
+        }
+        rule.declarations ??= [];
+        for (const declaration of defaultRule.declarations ?? []) {
+          if (declaration.type !== "declaration") continue;
+          if (!rule.declarations.some((d) => d.property === declaration.property)) {
+            rule.declarations.push(declaration);
+            completed = true;
+          }
+        }
+      }
+    }
+    if (completed) this._saveCompletedCss(cssFile);
+  }
+
+  /**
+   * Writes the completed AST to the user's stylesheet, after copying the file to
+   * stylesheet.css.bak.
+   * @param {Gio.File} cssFile
+   */
+  _saveCompletedCss(cssFile) {
+    try {
+      const backup = Gio.File.new_for_path(`${cssFile.get_path()}.bak`);
+      const flags = Gio.FileCopyFlags.OVERWRITE | Gio.FileCopyFlags.TARGET_DEFAULT_PERMS;
+      cssFile.copy(backup, flags, null, null);
+      cssFile.replace_contents(
+        stringify(this.cssAst),
+        null,
+        false,
+        Gio.FileCreateFlags.REPLACE_DESTINATION,
+        null
+      );
+      console.warn(
+        `Forge: completed the stylesheet ${cssFile.get_path()} (the previous one is ${backup.get_path()})`
+      );
+    } catch (e) {
+      console.warn(`Forge: can't save the stylesheet ${cssFile.get_path()}: ${e.message}`);
+    }
+  }
+
+  /**
+   * Parses a stylesheet file, or returns null (with a warning) if it can't be read or parsed.
+   * @param {Gio.File} cssFile
+   */
+  _parseCssFile(cssFile) {
+    try {
+      const [success, contents] = cssFile.load_contents(null);
+      if (success) return parse(new TextDecoder().decode(contents));
+    } catch (e) {
+      console.warn(`Forge: can't use the stylesheet ${cssFile.get_path()}: ${e.message}`);
     }
+    return null;
   }
 
   /**