How to change the translucent watermark in the tagline

Hi there,

In the tagline, the default setting displays the first letter of title. However, it does not work for Chinese characters. It displays a question mark when I use Chinese in the tagline. I intend to change the question mark into my own logo. where can I change that code?

here is the website:https://perksummit.com/nova/aboutsummit/