Make sure alerts use default font color

If we don't unset it, the default bootstrap p style turns
dark mode illegible (uses white-ish color). This seems
to be an issue specific to the very old version of Bootstrap
being used (v3.x).

When running the site locally, I could not find any other
.css (or .less) files being built or linked from the page,
so I placed the only needed style in the only other style
file other than bootstrap.

src/NuGetGallery/Content/gallery/css/fabric.css

@@ -7,6 +7,10 @@
 /*
   Your use of the content in the files referenced here is subject to the terms of the license at http://aka.ms/fabric-assets-license
 */
+.markdown-alert > p {
+    color: unset !important;
+}
+
 @keyframes ms-motion-fadeIn {
   from {
     opacity: 0;