This is how you can make the Login Form transparent:
1. Add this css to Server Module-> Custom css:
.frmLogin .x-window-body { background-color: transparent !important; }
2. Make the form color in design time to 'clNone'.
3. Form Login->ClientEvents->UniEvents->BeforeInit:
function window.beforeInit(sender, config) { config.baseCls='frmLogin'; }
It will make totally transparent with no border, only controls will appear.
I use Extjs 6.5