Theme arc-green: reverse heatmap colors (#8840)

* Theme arc-green: reverse heatmap colors

This uses the same colors as the updated palette in the base theme.

See #8709 and #5864, in particular [my comment showing the problem](https://github.com/go-gitea/gitea/issues/5864#issuecomment-462334171)

* Rebuild CSS

* Use link color as hot, interpolate between hot and cold colors

* Use color from a:hover

public/less/themes/arc-green.less

@@ -1294,8 +1294,34 @@ a.ui.labels .label:hover {
     }
 }
 
+.heatmap(@heat) {
+    @heatmap-cold: #2d303b;
+    @heatmap-hot: #a0cc75;
+    background-color: mix(@heatmap-hot, @heatmap-cold, @heat);
+}
+
 .heatmap-color-0 {
-    background-color: #2d303b;
+    .heatmap(0%);
+}
+
+.heatmap-color-1 {
+    .heatmap(20%);
+}
+
+.heatmap-color-2 {
+    .heatmap(40%);
+}
+
+.heatmap-color-3 {
+    .heatmap(60%);
+}
+
+.heatmap-color-4 {
+    .heatmap(80%);
+}
+
+.heatmap-color-5 {
+    .heatmap(100%);
 }
 
 /* code mirror dark theme */