Better design on web app (HTML, CSS and others)

Hi UX / designers!

I need to improove a better good looking on our web applcation system. Actually I used bootstrap framework. It working but we need to make an easy user interface to use.

We pretend to keep the structure of pages. (Main page and Iframes) And we need to change CSS and html only.

I'm talking about this layout:

https://loja.zionpdv.com.br/

login: c a i o r o d r o @ g m a i l . c o m (*without spaces)

pass: zion

Be gentil with sugestions on access in this link and give me some solutions, please.

Thanks!