xterm scrollbar styles

terminus-terminal/src/frontends/xterm.css

+.xterm-viewport::-webkit-scrollbar {
+    background: rgba(0, 0, 0, .125);
+}
+
+.xterm-viewport::-webkit-scrollbar-thumb {
+    background: rgba(255, 255, 255, .25);
+}

terminus-terminal/src/frontends/xtermFrontend.ts

@@ -3,6 +3,7 @@ import { Terminal, ITheme } from 'xterm'
 import * as fit from 'xterm/lib/addons/fit/fit'
 import * as ligatures from 'xterm-addon-ligatures-tmp'
 import 'xterm/dist/xterm.css'
+import './xterm.css'
 import deepEqual = require('deep-equal')
 
 Terminal.applyAddon(fit)