Use the same text color throughout the app

Change-Id: I55f83e0dbdd83efcbe179683b87060440908e789
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
index 9fd564c..008a85b 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html
@@ -83,7 +83,7 @@
       .diff-row.target-row.target-side-right .lineNum.right,
       .diff-row.target-row.unified .lineNum {
         background-color: #BBDEFB;
-        color: #000;
+        color: var(--primary-text-color);
       }
       .blank,
       .content {