[core] Fix Blockquote Style (#148)

This commit fixes the style of blockquotes when we render the item
description as Markdown. Until now blockquotes had a weird blue
background, so that the text wasn't readable.

Now we are using the secondary color as background and we add a border on
the left site with the primary color.

We would also like to use italic as font family, but there is currently
a bug, so that the defined blockquote style is not applied. See
https://github.com/flutter/flutter/issues/81720

app/lib/widgets/item/details/utils/item_description.dart

@@ -54,6 +54,15 @@ class ItemDescription extends StatelessWidget {
         codeblockDecoration: const BoxDecoration(
           color: Constants.secondary,
         ),
+        blockquoteDecoration: const BoxDecoration(
+          color: Constants.secondary,
+          border: Border(
+            left: BorderSide(
+              color: Constants.primary,
+              width: 1,
+            ),
+          ),
+        ),
       ),
       onTapLink: (text, href, title) {
         if (href != null) {

app/lib/widgets/item/preview/utils/item_description.dart

@@ -55,6 +55,15 @@ class ItemDescription extends StatelessWidget {
           codeblockDecoration: const BoxDecoration(
             color: Constants.secondary,
           ),
+          blockquoteDecoration: const BoxDecoration(
+            color: Constants.secondary,
+            border: Border(
+              left: BorderSide(
+                color: Constants.primary,
+                width: 1,
+              ),
+            ),
+          ),
         ),
         onTapLink: (text, href, title) {
           if (href != null) {