Looking into the Style Panel, I made the change to the colour setting (within the Background settings section) as opposed changing the setting for Image & Gradient (Add Background Image, Add Gradient, Add Solid Colour).
I’ve highlighted the setting in the image below.
That will allow you to style as intended and apply the class btn2 to any additional buttons you use.
A second option would be to style the default ‘Button’ class with default styling settings you want and apply the ‘Button’ class to future buttons added.
To style the ‘Button’ class, after adding the button component from the Elements panel, select the ‘Button’ class and make your styling changes.
Hope htat helps.