Create basic theme service

Part of #4184

addons/xterm-addon-webgl/src/renderLayer/CursorRenderLayer.ts

@@ -7,7 +7,7 @@ import { Terminal } from 'xterm';
 import { BaseRenderLayer } from './BaseRenderLayer';
 import { ICellData } from 'common/Types';
 import { CellData } from 'common/buffer/CellData';
-import { IColorSet } from 'browser/Types';
+import { IColorSet, ReadonlyColorSet } from 'browser/Types';
 import { IRenderDimensions, IRequestRedrawEvent } from 'browser/renderer/shared/Types';
 import { IEventEmitter } from 'common/EventEmitter';
 import { ICoreBrowserService } from 'browser/services/Services';
@@ -37,7 +37,7 @@ export class CursorRenderLayer extends BaseRenderLayer {
     terminal: Terminal,
     container: HTMLElement,
     zIndex: number,
-    colors: IColorSet,
+    colors: ReadonlyColorSet,
     private _onRequestRefreshRowsEvent: IEventEmitter<IRequestRedrawEvent>,
     coreBrowserService: ICoreBrowserService,
     private readonly _coreService: ICoreService