[SalesForce] modify the Icon in a Salesforce1 Custom Button accessed Visualforce Page

From what I understand, Salesforce1 is using the Salesforce Mobile SDK which is built on top of Phonegap (see Andrew's informative answer here for more on the technologies behind the Salesforce1 app).

So taking a look at what Phonegap has to say on the matter:

The web view used by PhoneGap is the same web view used by the native operating system. On iOS, this is the Objective-C UIWebView class; on Android, this is android.webkit.WebView. Since there are differences in the web view rendering engines between operating systems, make sure that you account for this in your UI implementation.

Unfortunately there isn't currently a way to assign an action to a button. sforce.one.createRecord() uses the standard page layout for the profile/record type, not an action layout.

You could fake it out by creating a Visualforce page that dynamically generates the form based on the action layout, something like this (this is a lead because it's code I had on hand, you can do something similar for your New Event action):

public class QuickCreateController{
    public final ApexPages.StandardController stdController;
    public Lead newLead = new Lead();
    public Map<string, boolean> requiredMap {get;set;}

    public QuickCreateController() {}

    public QuickCreateController(ApexPages.StandardController stdController) 
    { 
        this.stdController = stdController;
        this.newLead = (Lead) stdController.getRecord();

        // Workaround for DescribeLayoutItem.required attribute apparently not
        // accessible in VF page (bug?): storing the required flags in a map.
        requiredMap = new Map<string, boolean>();
    }

    // Get the list of fields in the NewLead global action.
    public List<QuickAction.DescribeLayoutItem> getActionFieldList()
    {
        List<QuickAction.DescribeLayoutItem> fieldList = new List<QuickAction.DescribeLayoutItem>();
        QuickAction.DescribeQuickActionResult[] results = QuickAction.describeQuickActions(new String[]{ QuickAction.NewLead });

        for (QuickAction.DescribeQuickActionResult res : results)
        {
            for (QuickAction.DescribeLayoutRow row : res.getLayout().getLayoutRows())
            {
                for (QuickAction.DescribeLayoutItem col : row.getLayoutItems())
                {
                    // Ignore placeholders.
                    if (!col.placeholder)
                    {
                        fieldList.add(col);

                        // Hack to get DescribeLayoutItem.required flag in the VF page: 
                        // store it in a map, request it from the page.
                        requiredMap.put(col.label, col.required);
                    }                        
                }
            }
        }

        return fieldList;
    }

    public PageReference submitQuickAction()
    {
        PageReference p = new PageReference('/apex/QuickCreate');
        QuickAction.QuickActionRequest req = new QuickAction.QuickActionRequest();
        req.quickActionName = QuickAction.NewLead;
        req.record = newLead;

        try 
        {
            QuickAction.QuickActionResult res = QuickAction.performQuickAction(req);
            p = new PageReference('/apex/Page_To_Go_To_After_Completion);
            p.setRedirect(true);
        }
        catch (Exception ex)
        {
            ApexPages.Message myMsg = new ApexPages.Message(ApexPages.Severity.ERROR, ex.getMessage());
            ApexPages.addMessage(myMsg);
        }

        return p;
    }
}

Then your Visualforce page looks something like this:

<apex:page standardController="Lead" extensions="QuickCreateController">
<apex:form >
    <apex:pageMessages />
    <apex:repeat value="{!actionFieldList}" var="layoutItem">
        <apex:outputPanel layout="none" rendered="{!requiredMap[layoutItem.label]}"><span style="font-weight: bold; color: red">*</span> </apex:outputPanel>
        <apex:outputLabel value="{!layoutItem.label}"></apex:outputLabel><br />
        <apex:repeat value="{!layoutItem.layoutComponents}" var="layoutComponent">
            <apex:inputField value="{!Lead[layoutComponent.value]}" required="{!requiredMap[layoutItem.label]}"/>
        </apex:repeat>
    </apex:repeat>
    <apex:commandButton action="{!submitQuickAction}" value="Save" />
</apex:form>
</apex:page>

It's a hack and I don't recommend it as a solution, but I thought it was interesting to know it could be done. :-)