Use css styles from admin LTE template or bootstrap. Invent your own styles only if really necessary.
If you invent your own styles the midpoint-theme.less file is the right place to put them. Make sure to correctly use LESS color variables.
If you use any icon anywhere, check whether there is already icon for that in GuiStyleConstants. If it is not then add it to the GuiStyleConstants and use the constant in your code. Do NOT use string literals for icons.