Theme arc-green: reverse heatmap colors

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

See go-gitea#8709 and go-gitea#5864, in particular [my comment showing the problem](go-gitea#5864 (comment))

Author: cdlm

public/less/themes/arc-green.less

@@ -1298,6 +1298,26 @@ a.ui.labels .label:hover {
     background-color: #2d303b;
 }
 
+.heatmap-color-1 {
+    background-color: #025900;
+}
+
+.heatmap-color-2 {
+    background-color: #609926;
+}
+
+.heatmap-color-3 {
+    background-color: #66c74b;
+}
+
+.heatmap-color-4 {
+    background-color: #9fdb81;
+}
+
+.heatmap-color-5 {
+    background-color: #d8efbf;
+}
+
 /* code mirror dark theme */
 
 .CodeMirror {