If your theme doesn't support the button WordPress File Upload plugin of WordPress then you can copy and try one by one both the CSS code and check, One will definitely 

CSS CODE NUMBER 1

input#input_1 {
    background: #4CAF50; /* Green */
    border: none;
    color: white;
    padding: 7px 32px;
    text-align: center;
    text-decoration: none;
    display: inline-block;
    font-size: 16px;
}

input#upload_1 {
    background: #4CAF50; /* Green */
    border: none;
    color: white;
    padding: 7px 32px;
    text-align: center;
    text-decoration: none;
    display: inline-block;
    font-size: 16px;
}

CSS CODE NUMBER 2

input#input_1 {
    background-color: #4CAF50; /* Green */
    border: none;
    color: white;
    padding: 7px 32px;
    text-align: center;
    text-decoration: none;
    display: inline-block;
    font-size: 16px;
}

input#upload_1 {
    background-color: #4CAF50; /* Green */
    border: none;
    color: white;
    padding: 7px 32px;
    text-align: center;
    text-decoration: none;
    display: inline-block;
    font-size: 16px;
}

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