In case anyone is still interested in this topic, you might need to know some basic Chinese for this or use Google translate but... this is the official site for 360 launcher themes: zhushou.mobile.360.cn just add http in front of it.
Some quick translations, the top orange menu bar with the four links up top from left to right:
1) 首页 - Home
The orange categories starting from under the banner:
- Limited Time Free Themes
- Popular DIY Themes
- DIY Themes by Author
- Free Themes
- Limited Time Trial Themes
- Theme Sets
2) DIY 主题 - DIY Themes
3) DIY 工具 - DIY Tool (Build your own theme)
4) 我的DIY主题 - My DIY Themes
I think the themes you make/download here will be downloaded as an apk file which you just install.