Add more theme.

resource/vditor/css/theme/Monokai.css

@@ -0,0 +1,11 @@
+:root {
+    --hr-bg: #49483e;
+    --ext-border-color: #49483e;
+    --bg-color: #272822;
+    --front-color: #f8f8f2;
+    --second-bg-color: #1e1f1c;
+    --code-bg-color: #3e3d32;
+    --list-hover-color: #66d9ef;
+    --dropdown-hover-background: #49483e;
+    --error-color: #f92672;
+} 

resource/vditor/css/theme/Nord.css

@@ -0,0 +1,11 @@
+:root {
+    --hr-bg: #4c566a;
+    --ext-border-color: #3b4252;
+    --bg-color: #2e3440;
+    --front-color: #d8dee9;
+    --second-bg-color: #3b4252;
+    --code-bg-color: #434c5e;
+    --list-hover-color: #88c0d0;
+    --dropdown-hover-background: #434c5e;
+    --error-color: #bf616a;
+} 

resource/vditor/css/theme/One Dark.css

@@ -0,0 +1,11 @@
+:root {
+    --hr-bg: #3f4451;
+    --ext-border-color: #181a1f;
+    --bg-color: #282c34;
+    --front-color: #abb2bf;
+    --second-bg-color: #21252b;
+    --code-bg-color: #31363f;
+    --list-hover-color: #61afef;
+    --dropdown-hover-background: #3a3f4b;
+    --error-color: #e06c75;
+} 

src/provider/markdownEditorProvider.ts

@@ -125,7 +125,12 @@ export class MarkdownEditorProvider implements vscode.CustomTextEditorProvider {
             new MarkdownService(this.context).exportMarkdown(uri, option)
         }).on("theme", async (theme) => {
             if (!theme) {
-                const themes = ["Auto", "|", "Light", "Solarized", "|", "Dracula", "Github Dark"]
+                const themes = [
+                    "Auto", "|",
+                    "Light", "Solarized", "|",
+                    "One Dark", "Github Dark",
+                    "Nord", "Monokai", "Dracula",
+                ];
                 const editorTheme = Global.getConfig('editorTheme');
                 const themeItems: vscode.QuickPickItem[] = themes.map(theme => {
                     if (theme == '|') return { label: '|', kind: vscode.QuickPickItemKind.Separator }