I feel like I'm missing some obvious... 

Here is a Lighting test component based on https://www.lightningdesignsystem.com/components/data-tables#responsive-stacked-horizontal. 
 

<aura:component implements="flexipage:availableForAllPageTypes, force:appHostable">
    <ltng:require styles="/resource/SLDS0101/assets/styles/salesforce-lightning-design-system-ltng.css" />
    <table class="slds-table slds-table--bordered slds-max-medium-table--stacked-horizontal">
        <thead>
            <tr class="slds-text-heading--label">
                <th class="slds-row-select" scope="col">
                    <label class="slds-checkbox" for="select-all">
                        <input name="checkbox" type="checkbox" id="select-all" />
                        <span class="slds-checkbox--faux"></span>
                        <span class="slds-form-element__label slds-assistive-text">select all</span>
                    </label>
                </th>
                <th class="slds-is-sortable" scope="col">
                    <span class="slds-truncate">Opportunity Name</span>
                    <button class="slds-button slds-button--icon-bare">
                        <span class="slds-assistive-text">Sort</span>
                    </button>
                </th>
                <th scope="col">
                    <span class="slds-truncate">Account Name</span>
                </th>
                <th scope="col">
                    <span class="slds-truncate">Close Date</span>
                </th>
                <th scope="col">
                    <span class="slds-truncate">Stage</span>
                </th>
                <th scope="col">
                    <span class="slds-truncate">Confidence</span>
                </th>
                <th scope="col">
                    <span class="slds-truncate">Amount</span>
                </th>
                <th scope="col">
                    <span class="slds-truncate">Contact</span>
                </th>
                <th class="slds-row-action" scope="col">
                    <button class="slds-button slds-button--icon-border-filled slds-button--icon-x-small">
                        <span class="slds-assistive-text">Show More</span>
                    </button>
                </th>
            </tr>
        </thead>
        <tbody>
            <tr class="slds-hint-parent">
                <td class="slds-row-select">
                    <label class="slds-checkbox" for="select-row1">
                        <input name="select-row1" type="checkbox" id="select-row1" />
                        <span class="slds-checkbox--faux"></span>
                        <span class="slds-form-element__label slds-assistive-text">select row1</span>
                    </label>
                </td>
                <th data-label="opportunity-name" role="row"><a href="#" class="slds-truncate">Acme 25</a></th>
                <td data-label="account"><a href="#" class="slds-truncate">Acme</a></td>
                <td data-label="activity">
                    <span class="slds-truncate">4/14/2015</span>
                </td>
                <td data-label="stage">
                    <span class="slds-truncate">Prospecting</span>
                </td>
                <td data-label="confidence">
                    <span class="slds-truncate">20%</span>
                </td>
                <td data-label="amount">
                    <span class="slds-truncate">$25k</span>
                </td>
                <td data-label="contact">
                    <span class="slds-truncate">--</span>
                </td>
                <td class="slds-row-action">
                    <button class="slds-button slds-button--icon-border-filled slds-button--icon-x-small">
                        <span class="slds-assistive-text">Show More</span>
                    </button>
                </td>
            </tr>
            <tr class="slds-hint-parent">
                <td class="slds-row-select">
                    <label class="slds-checkbox" for="select-row2">
                        <input name="select-row2" type="checkbox" id="select-row2" />
                        <span class="slds-checkbox--faux"></span>
                        <span class="slds-form-element__label slds-assistive-text">select row2</span>
                    </label>
                </td>
                <th data-label="opportunity-name" role="row"><a href="#" class="slds-truncate">Rohde Corp 30</a></th>
                <td data-label="account"><a href="#" class="slds-truncate">Rohde Corp</a></td>
                <td data-label="activity">
                    <span class="slds-truncate">6/18/2015</span>
                </td>
                <td data-label="stage">
                    <span class="slds-truncate">Prospecting</span>
                </td>
                <td data-label="confidence">
                    <span class="slds-truncate">40%</span>
                </td>
                <td data-label="amount">
                    <span class="slds-truncate">$30k</span>
                </td>
                <td data-label="contact"><a href="#" class="slds-truncate">achoi@rohdecorp.com</a></td>
                <td class="slds-row-action">
                    <button class="slds-button slds-button--icon-border-filled slds-button--icon-x-small">
                        <span class="slds-assistive-text">Show More</span>
                    </button>
                </td>
            </tr>
        </tbody>
    </table>
</aura:component>

Create an app like:
 

<aura:application >
<ltng:require styles="/resource/SLDS0101/assets/styles/salesforce-lightning-design-system-ltng.css" />
    <div class="slds" style="margin-top:10px;margin-left:10px;margin-right:10px">
        <c:CSSTest />
    </div>
</aura:application>

... loads up on my desktop browser using "Preview" from the developer console, no problem. All responsive and styled.

However, when I add this using the Lighting App Building to make a new "app" for Salesforce1, it loses the Lightning Design System styles. Any idea what I'm missing and/or doing wrong? Is there an embedded "salesforce-lightning-design-system-ltng.css" in Salesforce1 that I am supposed to reference? Or am I supposed to do something else fancy?

From what I can tell none of the tutorials on Trailhead or elsewhere cover this, so  any advice is most appreciated.

Thanks!