Use the same text color throughout the app

Change-Id: I55f83e0dbdd83efcbe179683b87060440908e789

polygerrit-ui/app/elements/change/gr-message/gr-message.html

@@ -138,7 +138,7 @@ limitations under the License.
       .score {
         border: 1px solid var(--border-color);
         border-radius: 3px;
-        color: #000;
+        color: var(--primary-text-color);
         display: inline-block;
         margin: -.1em 0;
         padding: 0 .1em;

polygerrit-ui/app/elements/diff/gr-diff-comment/gr-diff-comment.html

@@ -169,7 +169,7 @@ limitations under the License.
         display: none;
       }
       label.show-hide {
-        color: #000;
+        color: var(--primary-text-color);
         cursor: pointer;
         display: block;
         font-size: .8rem;

polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.html

@@ -182,7 +182,7 @@ limitations under the License.
           vertical-align: -.1em;
         }
         .mobileNavLink {
-          color: #000;
+          color: var(--primary-text-color);
           font-size: 1.5rem;
           font-family: var(--font-family-bold);
           text-decoration: none;

polygerrit-ui/app/elements/diff/gr-diff/gr-diff.html

@@ -83,7 +83,7 @@ limitations under the License.
       .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 {

polygerrit-ui/app/elements/shared/gr-download-commands/gr-download-commands.html

@@ -44,7 +44,7 @@ limitations under the License.
         font-family: var(--font-family-bold);
       }
       li[selected] gr-button {
-        color: #000;
+        color: var(--primary-text-color);
         font-family: var(--font-family-bold);
         text-decoration: none;
       }

polygerrit-ui/app/styles/gr-change-list-styles.html

@@ -84,7 +84,7 @@ limitations under the License.
         background-color: #eaeaea;
       }
       .groupHeader a {
-        color: #000;
+        color: var(--primary-text-color);
         text-decoration: none;
       }
       .groupHeader a:hover {