A simple CSS overwrite to add old-style gradients to Bootstrap 3 flat buttons. CSS created from http://twitterbootstrap3buttons.w3masters.nl

twbs-btn-gradients

.btn-primary {
    background-color: #08c;
    border-color: #08c;
    color: #fff;
    background-color: #007ab8;
    background-image: -moz-linear-gradient(top, #08c, #006699);
    background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#08c), to(#006699));
    background-image: -webkit-linear-gradient(top, #08c, #006699);
    background-image: -o-linear-gradient(top, #08c, #006699);
    background-image: linear-gradient(to bottom, #08c, #006699);
    background-repeat: repeat-x;
    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ff0088cc', endColorstr='#ff006599', GradientType=0);
    border-color: #006699 #006699 #00334d;
    border-color: rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.25);
}

.btn-primary:hover,
.btn-primary:focus {
    text-decoration: none;
    background-position: 0 -15px;
    -webkit-transition: background-position .1s linear;
    -moz-transition: background-position .1s linear;
    -o-transition: background-position .1s linear;
    transition: background-position .1s linear;
}

.btn-primary:hover,
.btn-primary:focus {
    background-color: #006699;
}

.btn-primary.disabled:hover,
.btn-primary.disabled:focus,
.btn-primary.disabled:active,
.btn-primary.disabled.active,
.btn-primary[disabled] .btn-primary:hover,
.btn-primary[disabled] .btn-primary:focus,
.btn-primary[disabled] .btn-primary:active,
.btn-primary[disabled] .btn-primary.active {
    background-color: #08c;
    border-color: #08c;
}

.btn-success {
    background-color: #5cb85c;
    border-color: #5cb85c;
    color: #fff;
    background-color: #53ad53;
    background-image: -moz-linear-gradient(top, #5cb85c, #449d44);
    background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#5cb85c), to(#449d44));
    background-image: -webkit-linear-gradient(top, #5cb85c, #449d44);
    background-image: -o-linear-gradient(top, #5cb85c, #449d44);
    background-image: linear-gradient(to bottom, #5cb85c, #449d44);
    background-repeat: repeat-x;
    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ff5cb85c', endColorstr='#ff449c44', GradientType=0);
    border-color: #449d44 #449d44 #2d672d;
    border-color: rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.25);
}

.btn-success:hover,
.btn-success:focus {
    text-decoration: none;
    background-position: 0 -15px;
    -webkit-transition: background-position .1s linear;
    -moz-transition: background-position .1s linear;
    -o-transition: background-position .1s linear;
    transition: background-position .1s linear;
}

.btn-success:hover,
.btn-success:focus {
    background-color: #449d44;
}

.btn-success.disabled:hover,
.btn-success.disabled:focus,
.btn-success.disabled:active,
.btn-success.disabled.active,
.btn-success[disabled] .btn-success:hover,
.btn-success[disabled] .btn-success:focus,
.btn-success[disabled] .btn-success:active,
.btn-success[disabled] .btn-success.active {
    background-color: #5cb85c;
    border-color: #5cb85c;
}

.btn-warning {
    background-color: #f0ad4e;
    border-color: #f0ad4e;
    color: #fff;
    background-color: #eea43b;
    background-image: -moz-linear-gradient(top, #f0ad4e, #ec971f);
    background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#f0ad4e), to(#ec971f));
    background-image: -webkit-linear-gradient(top, #f0ad4e, #ec971f);
    background-image: -o-linear-gradient(top, #f0ad4e, #ec971f);
    background-image: linear-gradient(to bottom, #f0ad4e, #ec971f);
    background-repeat: repeat-x;
    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#fff0ad4e', endColorstr='#ffec971e', GradientType=0);
    border-color: #ec971f #ec971f #b06d0f;
    border-color: rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.25);
}

.btn-warning:hover,
.btn-warning:focus {
    text-decoration: none;
    background-position: 0 -15px;
    -webkit-transition: background-position .1s linear;
    -moz-transition: background-position .1s linear;
    -o-transition: background-position .1s linear;
    transition: background-position .1s linear;
}

.btn-warning:hover,
.btn-warning:focus {
    background-color: #ec971f;
}

.btn-warning.disabled:hover,
.btn-warning.disabled:focus,
.btn-warning.disabled:active,
.btn-warning.disabled.active,
.btn-warning[disabled] .btn-warning:hover,
.btn-warning[disabled] .btn-warning:focus,
.btn-warning[disabled] .btn-warning:active,
.btn-warning[disabled] .btn-warning.active {
    background-color: #f0ad4e;
    border-color: #f0ad4e;
}

.btn-danger {
    background-color: #d9534f;
    border-color: #d9534f;
    color: #fff;
    background-color: #d34541;
    background-image: -moz-linear-gradient(top, #d9534f, #c9302c);
    background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#d9534f), to(#c9302c));
    background-image: -webkit-linear-gradient(top, #d9534f, #c9302c);
    background-image: -o-linear-gradient(top, #d9534f, #c9302c);
    background-image: linear-gradient(to bottom, #d9534f, #c9302c);
    background-repeat: repeat-x;
    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ffd9534f', endColorstr='#ffc9302b', GradientType=0);
    border-color: #c9302c #c9302c #8b211e;
    border-color: rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.25);
}

.btn-danger:hover,
.btn-danger:focus {
    text-decoration: none;
    background-position: 0 -15px;
    -webkit-transition: background-position .1s linear;
    -moz-transition: background-position .1s linear;
    -o-transition: background-position .1s linear;
    transition: background-position .1s linear;
}

.btn-danger:hover,
.btn-danger:focus {
    background-color: #c9302c;
}

.btn-danger.disabled:hover,
.btn-danger.disabled:focus,
.btn-danger.disabled:active,
.btn-danger.disabled.active,
.btn-danger[disabled] .btn-danger:hover,
.btn-danger[disabled] .btn-danger:focus,
.btn-danger[disabled] .btn-danger:active,
.btn-danger[disabled] .btn-danger.active {
    background-color: #d9534f;
    border-color: #d9534f;
}

.btn-info {
    background-color: #5bc0de;
    border-color: #5bc0de;
    color: #fff;
    background-color: #4ab9db;
    background-image: -moz-linear-gradient(top, #5bc0de, #31b0d5);
    background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#5bc0de), to(#31b0d5));
    background-image: -webkit-linear-gradient(top, #5bc0de, #31b0d5);
    background-image: -o-linear-gradient(top, #5bc0de, #31b0d5);
    background-image: linear-gradient(to bottom, #5bc0de, #31b0d5);
    background-repeat: repeat-x;
    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ff5bc0de', endColorstr='#ff30afd5', GradientType=0);
    border-color: #31b0d5 #31b0d5 #1f7e9a;
    border-color: rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.25);
}

.btn-info:hover,
.btn-info:focus {
    text-decoration: none;
    background-position: 0 -15px;
    -webkit-transition: background-position .1s linear;
    -moz-transition: background-position .1s linear;
    -o-transition: background-position .1s linear;
    transition: background-position .1s linear;
}

.btn-info:hover,
.btn-info:focus {
    background-color: #31b0d5;
}

.btn-info.disabled:hover,
.btn-info.disabled:focus,
.btn-info.disabled:active,
.btn-info.disabled.active,
.btn-info[disabled] .btn-info:hover,
.btn-info[disabled] .btn-info:focus,
.btn-info[disabled] .btn-info:active,
.btn-info[disabled] .btn-info.active {
    background-color: #5bc0de;
    border-color: #5bc0de;
}

.btn-default {
    background-color: #fff;
    border-color: #fff;
    color: #333;
    background-color: #f5f5f5;
    background-image: -moz-linear-gradient(top, #fff, #e6e6e6);
    background-image: -webkit-gradient(linear, 0 0, 0 100%, from(#fff), to(#e6e6e6));
    background-image: -webkit-linear-gradient(top, #fff, #e6e6e6);
    background-image: -o-linear-gradient(top, #fff, #e6e6e6);
    background-image: linear-gradient(to bottom, #fff, #e6e6e6);
    background-repeat: repeat-x;
    filter: progid:DXImageTransform.Microsoft.gradient(startColorstr='#ffffffff', endColorstr='#ffe5e5e5', GradientType=0);
    border-color: #e6e6e6 #e6e6e6 #bfbfbf;
    border-color: rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.1) rgba(0, 0, 0, 0.25);
}

.btn-default:hover,
.btn-default:focus {
    text-decoration: none;
    background-position: 0 -15px;
    -webkit-transition: background-position .1s linear;
    -moz-transition: background-position .1s linear;
    -o-transition: background-position .1s linear;
    transition: background-position .1s linear;
}

.btn-default:hover,
.btn-default:focus {
    background-color: #e6e6e6;
}

.btn-default.disabled:hover,
.btn-default.disabled:focus,
.btn-default.disabled:active,
.btn-default.disabled.active,
.btn-default[disabled] .btn-default:hover,
.btn-default[disabled] .btn-default:focus,
.btn-default[disabled] .btn-default:active,
.btn-default[disabled] .btn-default.active {
    background-color: #fff;
    border-color: #fff;
}