[devtools] Improve Dark Theme third-party badges

Previously, the third-party badges in the Network pane weren't legible
with the DevTools Dark Theme enabled. This patch fixes that.

Screenshot: http://goo.gle/devtools-dark-3pb

BUG=1003942, 1004246

Change-Id: Idea50cd4fbe458d171fbd7468dd0394a7a28ef04
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1805658Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Mathias Bynens <mathias@chromium.org>
Cr-Commit-Position: refs/heads/master@{#696785}

third_party/blink/renderer/devtools/front_end/product_registry/badge.css

@@ -21,6 +21,10 @@
   display: none;
 }
 
+:host-context(.-theme-with-dark-background) .product-registry-badge::before {
+  color: #000;
+}
+
 .product-registry-badge-title {
   padding-left: 4px;
-}
+}
\ No newline at end of file