Color contrast - Individual announcement page

fixes CNVS-15673
fixes CNVS-15672

Since these two areas dealt with similar contrast issues, they're
tackled in one commit.

This adds the correct color ratios to meet accessibility.

Test Plan:

CNVS-15673

- Go to a specific discussion

- Click on "reply", make sure the ratio for the link "HTML Editor"
meets webaim standards. Note: we have taken away the blue background
purposely.
- Get the colors on the pubish timestamps; both on the main message
and on the individuals (see screenshots in jira story). These now
meet webaim ratio requirements.

CNVS-15672

- Go to announcements
- There should no longer be a gray background zebra striping on
some of the announcments, and this fixes the majority of ratio
contrast issues for HC mode (see jira ticket for screenshots)

Change-Id: Id76f1a6a34641af67237a0ec6298a4224786e922
Reviewed-on: https://gerrit.instructure.com/43814
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Vincent Gonzalez <vgonzalez@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Colleen Palmer <colleen@instructure.com>

app/stylesheets/pages/discussions.scss

@@ -1,8 +1,8 @@
-@import "base/environment";
 @import "components/ui.selectmenu";
 
 $border: #C1C7CF;
 $borderRadius: 3px;
+$discussion-dark-text: $canvas-dark;
 
 body.with-right-side.with-left-side {
   // toolbar needs it to be this wide
@@ -158,7 +158,7 @@ body.with-right-side.with-left-side {
 }
 
 .due_date_wrapper {
-  color: #777777;
+  color: $discussion-dark-text;
 }
 
 .topic {
@@ -188,12 +188,12 @@ body.with-right-side.with-left-side {
 }
 
 .discussion-points {
-  color: #777777;
+  color: $discussion-dark-text;
 }
 
 .discussion-pubdate {
   font-size: 11px;
-  color: #777777;
+  color: $discussion-dark-text;
   line-height: 23px;
 }
 
@@ -210,7 +210,7 @@ body.with-right-side.with-left-side {
 .discussion-fyi {
   font-style: italic;
   font-size: 12px;
-  color: #777777;
+  color: $discussion-dark-text;
 }
 
 .discussion-read-state-btn {
@@ -272,7 +272,7 @@ body.with-right-side.with-left-side {
 }
 
 .discussion-entry-reply-area {
-  background: #f4f5f7;
+  background: #fff;
   padding: 0 0.8em;
   border-top: 1px solid $border;
 }
@@ -655,7 +655,7 @@ body.with-right-side.with-left-side {
 }
 
 .discussion-topic-due-dates {
-  color: #777777;
+  color: $discussion-dark-text;
   margin: 16px 0;
   width: 100%;
   th {

app/stylesheets/pages/discussions_list.scss

@@ -12,7 +12,9 @@
     min-height: 52px;
     border-top: 1px solid $tableBorder;
     background-color: white;
-    &:nth-child(odd) { background-color: $tableBackgroundAccent; }
+    @if $use_high_contrast == false {
+      &:nth-child(odd) { background-color: $tableBackgroundAccent; }
+    }
     &.unread .discussion-title{ font-weight: bold; }
     &.selected { background-color: #d6ecfc; }
   }