Exclude Bison-generated files from coverage report, and use dark theme (#1742)

.github/workflows/coverage.yml

@@ -5,7 +5,7 @@ on:
 
 jobs:
   coverage:
-    runs-on: ubuntu-22.04
+    runs-on: ubuntu-24.04
     steps:
       - name: Checkout repo
         uses: actions/checkout@v4
@@ -27,4 +27,8 @@ jobs:
         uses: actions/upload-artifact@v4
         with:
           name: coverage-report
-          path: coverage
+          # Workaround for keeping the top-level coverage/ directory
+          # https://github.com/actions/upload-artifact/issues/174
+          path: |
+            coverage
+            dummy-file-to-keep-directory-structure.txt