Updated all dialogs to define their primary button, I've selected the one that had been specified for focus, but the primary button isn't dependent on focus, it's a UX feature

src/components/views/dialogs/ErrorDialog.js

@@ -59,7 +59,7 @@ module.exports = React.createClass({
                     {this.props.description}
                 </div>
                 <div className="mx_Dialog_buttons">
-                    <button onClick={this.props.onFinished} autoFocus={this.props.focus}>
+                    <button className="mx_Dialog_primary" onClick={this.props.onFinished} autoFocus={this.props.focus}>
                         {this.props.button}
                     </button>
                 </div>

src/components/views/dialogs/LogoutPrompt.js

@@ -46,7 +46,7 @@ module.exports = React.createClass({
                     Sign out?
                 </div>
                 <div className="mx_Dialog_buttons" onKeyDown={ this.onKeyDown }>
-                    <button autoFocus onClick={this.logOut}>Sign Out</button>
+                    <button className="mx_Dialog_primary" autoFocus onClick={this.logOut}>Sign Out</button>
                     <button onClick={this.cancelPrompt}>Cancel</button>
                 </div>
             </div>

src/components/views/dialogs/NeedToRegisterDialog.js

@@ -63,7 +63,7 @@ module.exports = React.createClass({
                     {this.props.description}
                 </div>
                 <div className="mx_Dialog_buttons">
-                    <button onClick={this.props.onFinished} autoFocus={true}>
+                    <button className="mx_Dialog_primary" onClick={this.props.onFinished} autoFocus={true}>
                         Cancel
                     </button>
                     <button onClick={this.onRegisterClicked}>

src/components/views/dialogs/QuestionDialog.js

@@ -56,7 +56,7 @@ module.exports = React.createClass({
                     {this.props.description}
                 </div>
                 <div className="mx_Dialog_buttons">
-                    <button onClick={this.onOk} autoFocus={this.props.focus}>
+                    <button className="mx_Dialog_primary" onClick={this.onOk} autoFocus={this.props.focus}>
                         {this.props.button}
                     </button>
 

src/components/views/dialogs/SetDisplayNameDialog.js

@@ -76,7 +76,7 @@ module.exports = React.createClass({
                         />
                     </div>
                     <div className="mx_Dialog_buttons">
-                        <input type="submit" value="Set" />
+                        <input className="mx_Dialog_primary" type="submit" value="Set" />
                     </div>
                 </form>
             </div>

src/components/views/dialogs/TextInputDialog.js

@@ -86,7 +86,7 @@ module.exports = React.createClass({
                     <button onClick={this.onCancel}>
                         Cancel
                     </button>
-                    <button onClick={this.onOk}>
+                    <button className="mx_Dialog_primary" onClick={this.onOk}>
                         {this.props.button}
                     </button>
                 </div>