You know the circle button that’s half colored in black at the top? Click that button, it will take you to your dashboard, click the “Theme Designer” option and you have three bars at the left, one for Theme, Customizing & Wordmarks, the “Page” section on the “Customize” bar shows the color schemes, you can go ahead and play around with those and see what you think looks good :)
As for the top image, you would need to go on the “Background” section for the header (on the “Customize” bar) and there’s a section called “Header Graphic”—you can insert an image there are it shall appear at the top. Just remember to crop it in the way you expect it to appear before putting it there, as your image may not appear as you wished.
That’s the first part, I’ll help you with the second later :)
Should I add her promotional images as Hi Suhyun and Bom&Hi? Like under another tabber? Also let me know if you have kakao talk so we can communicate better. Also, I can help you out on The Boyz Wiki. :)