static: improve dark mode UI for deprecations

Updates background colors for dark mode to reduce
eye strain.

Fixes golang/go#48679

Change-Id: I32e43cafa6712f62f9d8636dea4243019c97bb06
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/355190
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>

static/shared/color/color.css

@@ -29,7 +29,7 @@
   --purple: #542c7d;
   --slate: #253443; /* Footer background. */
   --white: #fff;
-  --yellow: #fddd00;
+  --yellow: #fceea5;
   --yellow-light: #fff8cc;
 
   /* Color Intents */

static/shared/color/color.md

@@ -67,6 +67,10 @@ adjust for dark mode.
   <go-color id="--blue"></go-color>
 </span>
 
+<span style="display:contents;">
+  <go-color id="--blue-light"></go-color>
+</span>
+
 <span style="display:contents;">
   <go-color id="--black"></go-color>
 </span>
@@ -75,6 +79,10 @@ adjust for dark mode.
   <go-color id="--green"></go-color>
 </span>
 
+<span style="display:contents;">
+  <go-color id="--green-light"></go-color>
+</span>
+
 <span style="display:contents;">
   <go-color id="--pink"></go-color>
 </span>
@@ -87,6 +95,10 @@ adjust for dark mode.
   <go-color id="--purple"></go-color>
 </span>
 
+<span style="display:contents;">
+  <go-color id="--purple-light"></go-color>
+</span>
+
 <span style="display:contents;">
   <go-color id="--slate"></go-color>
 </span>