Hi Tuanphan! Sorry for the late response, I wasn't notified so I didn't know you replied.
I used google chrome and safari, and they both are showing the yellow color on top of my header and underneath my footer. You have to drag the screen all the way up or down and the color will show. They don't stay there either after you let go of your finger, but the color always appears whenever I swiftly scrolls the screen. I know it's such a small thing but it bothers me so much!
Really hope you can help me solve this. Thank you so much!!