i tried to change the button for account confirmation email template in my custom theme like this:
app/design/frontend/Test/mine/web/css/source/_email-extend.less
.button{ a{ background-color: #00A73F; border: 1px solid #00A73F !important; border-radius: 10px !important; -webkit-border-radius: 10px !important; -moz-border-radius: 10px !important; -ms-border-radius: 10px !important; &:active { text-decoration: none; border: 1px solid #00A73F !important; border-radius: 10px !important; -webkit-border-radius: 10px !important; -moz-border-radius: 10px !important; -ms-border-radius: 10px !important; } &:focus { text-decoration: none; border: 1px solid #00A73F !important; border-radius: 10px !important; -webkit-border-radius: 10px !important; -moz-border-radius: 10px !important; -ms-border-radius: 10px !important; } &:visited { text-decoration: none; border: 1px solid #00A73F !important; border-radius: 10px !important; -webkit-border-radius: 10px !important; -moz-border-radius: 10px !important; -ms-border-radius: 10px !important; } &:hover { text-decoration: none; border: 1px solid #00A73F !important; border-radius: 10px !important; -webkit-border-radius: 10px !important; -moz-border-radius: 10px !important; -ms-border-radius: 10px !important; } } } the button background color is successfully changed, but when i hover the button, the color back to the old color