[ReaderMode] Update background and text colours for themes

Updates background, text and link colours to match Figma mocks, and also
fix some colour contrast issues.

Bug: 1025300
Change-Id: I1fca11a04863718ffa1676fe978538a2ebacd6c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2114475Reviewed-by: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Commit-Queue: Meredith Lane <meredithl@chromium.org>
Cr-Commit-Position: refs/heads/master@{#753037}

components/dom_distiller/core/css/distilledpage.css

@@ -11,8 +11,12 @@
 
 :root {
   --google-blue-700: 25, 103, 210;
+  --google-blue-300: 138, 180, 248;
   --google-brown-900: 62, 39, 35;
+  --google-grey-50: 248, 249, 250;
+  --google-grey-900: 32, 33, 36;
   --google-purple-700: 132, 48, 206;
+  --google-purple-300: 197, 138, 249;
   --google-yellow-50: 254, 247, 224;
   --google-yellow-100: 254, 239, 195;
 }
@@ -111,13 +115,13 @@ body {
  * and with CSS class constants in viewer.cc */
 
 .light {
-  background-color: #FAFAFA;
-  color: #424242;
+  background-color: #FFF;
+  color: rgb(var(--google-grey-900));
 }
 
 .dark {
-  background-color: #212121;
-  color: #E0E0E0;
+  background-color: rgb(var(--google-grey-900));
+  color: #FFF;
 }
 
 .sepia {
@@ -126,7 +130,7 @@ body {
 }
 
 .light a:link {
-  color: rgb(85, 85, 255);
+  color: rgb(var(--google-blue-600));
 }
 
 .sepia a:link {
@@ -134,11 +138,11 @@ body {
 }
 
 .dark a:link {
-  color: rgb(136, 136, 255);
+  color: rgb(var(--google-blue-300));
 }
 
 .light a:visited {
-  color: rgb(144, 34, 144);
+  color: rgb(var(--google-purple-700));
 }
 
 .sepia a:visited {
@@ -146,13 +150,13 @@ body {
 }
 
 .dark a:visited {
-  color: rgb(216, 114, 216);
+  color: rgb(var(--google-purple-300));
 }
 
 .light code,
 .light pre {
-  background-color: #EEE;
-  border-color: #AAA;
+  background-color: rgb(var(--google-grey-50));
+  border-color: rgb(var(--google-grey-900));
 }
 
 .sepia code,
@@ -163,8 +167,8 @@ body {
 
 .dark code,
 .dark pre {
-  background-color: #333;
-  border-color: #555;
+  background-color: #000;
+  border-color: rgba(255, 255, 255, 0.5);
 }
 
 .light tbody tr:nth-child(odd) {