How can I make sidebar_first narrower? I try to make the sidebar_first narrower in a Bootstrap 3 theme in Drupal 8. There is no numbered class like col-sm-4 here that I could change and thus ask the devs. What is the right approach to take?

        &lt;div class="row"&gt;
            {% if page.sidebar_first %}
            &lt;aside class="{{sidebar_grid_class}}"&gt;  
                &lt;!--#sidebar-first--&gt;
                &lt;section id="sidebar-first" class="sidebar clearfix"&gt;
                {{ page.sidebar_first }}
                &lt;/section&gt;
                &lt;!--EOF:#sidebar-first--&gt;
            &lt;/aside&gt;
            {% endif %}
col-sm-4 will be applied via a function in the theme. if you search through the theme for where the variable is set you can change it there. Possibly in the .theme file.