How to set dark and light stock chart pages on the App:
On the [Market] page, click on any stock to enter the stock chart page, then click the [Settings] button at the bottom left of the chart, and then select the [Theme Mode] option, and you can set the dark and light stock chart page according to your personal preference.

How to set dark and light stock chart pages on the Web:
Open the official website, click [Trade] - [US Stocks] in the upper left corner, select any stock, and click the moon or sun icon in the upper right corner of the trading page to set the dark and light stock chart page.
