Refer to the documentation for creating custom theme CSS with appropriate changes (font-family):
body { background-color: #edeff2; color: #87919f; font-family: "Open Sans",Arial,sans-serif; font-size: 13px; line-height: 22px; margin: 0; padding: 30px 0 0; }
.Normal, .NormalDisabled, .NormalDeleted { color: #87919F; font-family: "Open Sans", Arial, sans-serif; font-size: 13px; line-height: 23px; }
h1, h2, h3, h4, h5, h6, .h1, .h2, .h3, .h4, .h5, .h6 { font-family: "Open Sans",Helvetica,Arial,sans-serif; font-weight: 500; line-height: 1.1; color: inherit; }