Hi Ou Yang Chia Lung,
Thank you for your purchase and contact us.
You needn't use the custom css code, The problem is caused by browser css cache.
If choose the let sidebar layout, some css code will be regenerated again according to the layout.
Please try like this.
1: Go to wp admin page->theme options and click the "save changes" button like screenshot.
Then, all css files will be regenerated.
2: Please go to your front page and press "ctrl+F5" to clear your css cache.
Then, you needn't to use the custom css code in there.
If you have any other questions, please let us know again.
Best Regards, C-Themes