feat: add system mode to the theme selector (#7853)

Co-authored-by: dwelle <5153846+dwelle@users.noreply.github.com>

packages/excalidraw/renderer/helpers.ts

@@ -2,7 +2,7 @@ import { StaticCanvasAppState, AppState } from "../types";
 
 import { StaticCanvasRenderConfig } from "../scene/types";
 
-import { THEME_FILTER } from "../constants";
+import { THEME, THEME_FILTER } from "../constants";
 
 export const fillCircle = (
   context: CanvasRenderingContext2D,
@@ -49,7 +49,7 @@ export const bootstrapCanvas = ({
   context.setTransform(1, 0, 0, 1, 0, 0);
   context.scale(scale, scale);
 
-  if (isExporting && theme === "dark") {
+  if (isExporting && theme === THEME.DARK) {
     context.filter = THEME_FILTER;
   }