Replace color: black with variable

This change also applies this variable in a few places -- due to shady
DOM style scoping, the value being applied to the body tag is not
enough to completely style the content.

Change-Id: Iebeb1833da399d3257be6dc38767280c2e38e209

polygerrit-ui/app/elements/change/gr-file-list-header/gr-file-list-header.html

@@ -105,7 +105,7 @@ limitations under the License.
         display: none;
       }
       gr-linked-chip {
-        --linked-chip-text-color: black;
+        --linked-chip-text-color: var(--primary-text-color);
       }
       .expanded #collapseBtn,
       .openFile .fileViewActions {

polygerrit-ui/app/elements/change/gr-label-score-row/gr-label-score-row.html

@@ -62,7 +62,7 @@ limitations under the License.
           @apply(--vote-chip-styles);
         }
         --gr-button-background: var(--button-background-color, #f5f5f5);
-        --gr-button-color: black;
+        --gr-button-color: var(--primary-text-color);
       }
       iron-selector > gr-button.iron-selected.max {
         --button-background-color: var(--vote-color-max);

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

@@ -43,6 +43,9 @@ limitations under the License.
 <dom-module id="gr-settings-view">
   <template>
     <style include="shared-styles">
+      :host {
+        color: var(--primary-text-color);
+      }
       #newEmailInput {
         width: 20em;
       }

polygerrit-ui/app/styles/gr-page-nav-styles.html

@@ -55,7 +55,7 @@ limitations under the License.
         font-family: var(--font-family-bold);
       }
       .navStyles a {
-        color: black;
+        color: var(--primary-text-color);
         display: inline-block;
         margin: .4em 0;
       }

polygerrit-ui/app/styles/shared-styles.html

@@ -92,7 +92,6 @@ limitations under the License.
         border-left: 1px solid rgba(0, 0, 0, .3);
         height: 20px;
         margin: 0 8px;
-
       }
       .separator.transparent {
         border-color: transparent;
@@ -104,6 +103,9 @@ limitations under the License.
       strong {
         font-family: var(--font-family-bold);
       }
+      :host {
+        color: var(--primary-text-color);
+      }
     </style>
   </template>
 </dom-module>