define fonts per platform

src/vs/workbench/electron-browser/media/shell.css

@@ -15,8 +15,12 @@
 
 /* Font Families (with CJK support) */
 
-.monaco-shell,
-.monaco-shell .monaco-menu-container .monaco-menu { font-family: -apple-system, BlinkMacSystemFont, "Segoe WPC", "Segoe UI", "HelveticaNeue-Light", "Ubuntu", "Droid Sans", sans-serif; }
+.monaco-shell.mac,
+.monaco-shell.mac .monaco-menu-container .monaco-menu { font-family: -apple-system, BlinkMacSystemFont, sans-serif; }
+.monaco-shell.windows,
+.monaco-shell.windows .monaco-menu-container .monaco-menu { font-family: "Segoe WPC", "Segoe UI", "HelveticaNeue-Light", sans-serif; }
+.monaco-shell.linux,
+.monaco-shell.linux .monaco-menu-container .monaco-menu { font-family: "Ubuntu", "Droid Sans", sans-serif; }
 .monaco-shell:lang(zh-Hans),
 .monaco-shell:lang(zh-Hans) .monaco-menu-container .monaco-menu { font-family: -apple-system, BlinkMacSystemFont, "Segoe WPC", "Segoe UI", "HelveticaNeue-Light", "Noto Sans", "Microsoft YaHei", "PingFang SC", "Hiragino Sans GB", "Source Han Sans SC", "Source Han Sans CN", "Source Han Sans", sans-serif; }
 .monaco-shell:lang(zh-Hant),
@@ -184,4 +188,4 @@
 .monaco-shell .monaco-tree.focused:focus,
 .monaco-shell .monaco-list:focus {
 	outline: 0 !important; /* tree indicates focus not via outline but through the focused item */
-}
\ No newline at end of file
+}

src/vs/workbench/electron-browser/shell.ts

@@ -87,7 +87,7 @@ import { NotificationService } from 'vs/workbench/services/notification/common/n
 import { IDialogService } from 'vs/platform/dialogs/common/dialogs';
 import { DialogService } from 'vs/workbench/services/dialogs/electron-browser/dialogService';
 import { DialogChannel } from 'vs/platform/dialogs/node/dialogIpc';
-import { EventType, addDisposableListener, addClass, scheduleAtNextAnimationFrame } from 'vs/base/browser/dom';
+import { EventType, addDisposableListener, scheduleAtNextAnimationFrame, addClasses } from 'vs/base/browser/dom';
 import { IRemoteAgentService } from 'vs/workbench/services/remote/node/remoteAgentService';
 import { RemoteAgentService } from 'vs/workbench/services/remote/electron-browser/remoteAgentServiceImpl';
 import { IOpenerService } from 'vs/platform/opener/common/opener';
@@ -538,7 +538,7 @@ export class WorkbenchShell extends Disposable {
 		});
 
 		// Shell Class for CSS Scoping
-		addClass(this.container, 'monaco-shell');
+		addClasses(this.container, 'monaco-shell', platform.isWindows ? 'windows' : platform.isLinux ? 'linux' : 'mac');
 
 		// Create Contents
 		this.renderContents();