searx/searx/static/themes/oscar/less/pointhi/search.less

.search_categories, #categories {
    text-transform: capitalize;
    margin-bottom: 1.5rem;
    margin-top: 1.5rem;
    display: flex;
    flex-wrap: wrap;
    align-content: stretch;

    label, .input-group-addon {
        flex-grow: 1;
        flex-basis: auto;
        font-size: 1.3rem;
        font-weight: normal;
        background-color: white;
        border: #DDD 1px solid;
        border-right: none;
        color: #333;
        padding-bottom: 0.8rem;
        padding-top: 0.8rem;
        text-align: center;
    }

    label:last-child, .input-group-addon:last-child {
        border-right: #DDD 1px solid;
    }

    input[type="checkbox"]:checked + label{
        color: black;
        font-weight: bold;
        background-color: #EEE;
    }
}