silverstripe-docsviewer/css/forms.css

fieldset {
    border: none;
    margin: 0;
    padding: 0 0 5px 0;
}

#DocumentationAdvancedSearchForm_AdvancedSearchForm_q {
    width: 100%;
    font-size: 15px;
    padding: 10px 10px;
}

#DocumentationAdvancedSearchForm_AdvancedSearchForm #q {
    width: 55%;
    padding-right: 2%;
    float: left;
}

#DocumentationAdvancedSearchForm_AdvancedSearchForm #Entities {
    width: 25%;
    padding-right: 2%;
    float: left;
}

#DocumentationAdvancedSearchForm_AdvancedSearchForm #Versions {
    width: auto;
    float: left;
    margin-top: 12px;
}

.optionset ul {
    margin: 0;
}
.optionset li {
    list-style: none;
    float: left;
    padding-right: 12px; 
}