Fix git graph dark theme text color issue

src/less/dark/gitGraph.less

@@ -0,0 +1,6 @@
+.commit-id,
+.commit-msg,
+.branch-label {
+  fill: lightgrey;
+  color: lightgrey;
+}

src/less/dark/mermaid.less

@@ -3,6 +3,7 @@
 @import "sequenceDiagram";
 @import "gantt";
 @import "classDiagram";
+@import "gitGraph";
 .node text {
   font-family: 'trebuchet ms', verdana, arial;
   font-size: 14px;