change the DifficultyPanel's size
simply design "HostOrClientPanel", add 2 buttons in it

btn1 : "Host a game"
btn2 : "Be a client"
add 2 new Panel into cardLayout

DifficultyPanel & HostOrClientPanel
set MainFrame into a static variable

in order to accessed by other panel & frame
setting "card" varaible to private memeber & add SwitchPanel methods

pass a string into SwitchPanel fuction and it can switch panels
add clicked events into 2 buttons

click "Single Player Mode" button to switch to DifficultyPanel
click "Network Battle Mode" button to switch to HostOrClientPanel