If you load the messagebox into chrome:devtools, you can see all the styles applied for what you want to change if you want to go at it directly.
For the icon, we use font-awesome in Ext6
http://docs.sencha.com/extjs/6.0/cor.../font_ext.html
Code:
.myMsgBox .x-window-body-default {
border-color: #5fa2dd;
border-width: 1px;
border-style: solid;
background: #ff0000;
color: #000;
font-size: 20px;
font-weight: 300;
font-family: 'Open Sans', 'Helvetica Neue', helvetica, arial, verdana, sans-serif;
}
Code:
Ext.Msg.show({
title:'Save Changes?',
message: 'Would you like to save your changes?',
buttons: Ext.Msg.YESNOCANCEL,
icon: Ext.Msg.QUESTION,
cls: 'myMsgBox'
});