button { background:none; border:none; border-radius:7px; }
/* background-image para formatar o gradiente e background-color para dar cor ao botão quando o navegador não suportar o background-image */ 
.bt-amarelo { color:#660; background-color: #ff3; background-image: linear-gradient(155deg, #ff3, #cc3); }
.bt-vermelho { color:#fff; background-color: #d00; background-image: linear-gradient(155deg, #d00, #800); }
.bt-azul { color:#fff; background-color: #55f; background-image: linear-gradient(155deg, #55f, #558); }
.bt-laranja { color:#fff; background-color: #f50; background-image: linear-gradient(155deg, #fc0, #c90); }
.bt-verde { color:#fff; background-color: #5b5; background-image: linear-gradient(155deg, #5b5, #383); }
.bt-roxo { color:#fff; background-color: #95f; background-image: linear-gradient(155deg, #95f, #50a); }
.bt-branco { color:#333; background-color: #f0f0f0; background-image: linear-gradient(155deg, #f0f0f0, #bababa); }
.bt-preto { color:#fff; background-color: #555; background-image: linear-gradient(155deg, #555, #000); }
.bt-cinza { color:#fff; background-color: #999; background-image: linear-gradient(155deg, #888, #555); }
.bt-desativado { color:#aaa; background-color: #ddd; background-image: linear-gradient(155deg, #ddd, #eee); }