weseek__growi/src/client/styles/scss/_button.scss

// fill button style
.btn-fill {
  position: relative;
  overflow: hidden;
  color: white;
  text-align: center;
  cursor: pointer;
  background-color: rgba(lighten(black, 20%), 0.4);
  border: none;

  .btn-label {
    position: relative;
    z-index: 1;
    color: white;
    text-decoration: none;
  }

  .btn-label-text {
    position: relative;
    z-index: 1;
    color: white;
    text-decoration: none;
  }

  // effect
  .eff {
    position: absolute;
    top: -50px;
    left: 0px;
    z-index: 0;
    width: 100%;
    height: 100%;
    transition: all 0.5s ease;
  }

  &:hover {
    .eff {
      top: 0;
    }
  }
}

// login
.btn-fill.login {
  .btn-label {
    background-color: rgba($danger, 0.4);
  }
  .eff {
    background-color: rgba(#7e4153, 0.5);
  }
}

// google
.btn-fill#google {
  .btn-label {
    background-color: rgba(#24292e, 0.4);
  }

  .eff {
    background-color: #555;
  }
}

// github
.btn-fill#github {
  .btn-label {
    background-color: rgba(lighten(black, 20%), 0.4);
  }

  .eff {
    background-color: #555;
  }
}

// facebook
.btn-fill#facebook {
  .btn-label {
    background-color: rgba(#29487d, 0.4);
  }

  .eff {
    background-color: #555;
  }
}

// twitter
.btn-fill#twitter {
  .btn-label {
    background-color: rgba(#1da1f2, 0.4);
  }

  .eff {
    background-color: #555;
  }
}

// oidc
.btn-fill#oidc {
  .btn-label {
    background-color: rgba(#24292e, 0.4);
  }

  .eff {
    background-color: #555;
  }
}

// saml
.btn-fill#saml {
  .btn-label {
    background-color: rgba(#55a79a, 0.4);
  }

  .eff {
    background-color: #555;
  }
}

// basic
.btn-fill#basic {
  .btn-label {
    background-color: rgba(#24292e, 0.4);
  }

  .eff {
    background-color: #555;
  }
}