_content/css: use color variables in download table

For the text in the download table to be visible for all
themes the row background color must switch with the
page theme.

For golang/go#34601

Change-Id: I369445f25efc3b3771f324f1c0555f9049cf3390
Reviewed-on: https://go-review.googlesource.com/c/website/+/398974
Trust: Jamal Carvalho <jamal@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

_content/css/styles.css

@@ -4467,7 +4467,7 @@ table.downloadtable {
   border-collapse: collapse;
 }
 table.downloadtable tr {
-  background-color: #f0f0f0;
+  background-color: var(--color-background-accented);
 }
 table.downloadtable tr:nth-child(2n), table.downloadtable tr.first {
   background-color: var(--color-background);