Hi,
Mobile first? Desktop first? Tablet first? NO. USERS FIRST!
Design first for the target audience your website is aimed at. Make it easy for the users to do what you want them to do. For example, do you want them to be amazed at the glorious parallax and animation effects on your website of do you want them to book a table?
Certainly, the stats show many use their mobiles, but by no means all. You should really ensure your design works for all users.
My Google metrics show mobile users make up two thirds of my customers, desktop next and about 5% tablets (which surprised me!). However, I try to make my website designs work equally well for all devices. Why wouldn’t you?
So, yes I always keep the mobile user in mind and design mobile first. Designing mobile first makes life a lot simpler in may ways. For instance, you can immediately forget about hover states (no pointer to activate them) and over elaborate animations (no much room for them). You concentrate on the content not the presentation.
The first thing with designing mobile first is that a new Sparkle project offers you the Desktop PCs device so I recommend that the very first thing you do is to enable the Portrait smartphones device. Alternatively, start with one of the many Sparkle templates, which usually have both these devicess enabled with sensible font choices, etc ready made.
The default view of the mobile device shows you how little room you have to work with! Zoom this view and it makes it easier to work with (but beware of the zoomed view leading you to use inappropriately sized objects and font sizes!).
I would urge you to gradually build your site, flip-floping between the devices, testing it in preview. If you don’t have a mobile and laptop/pc/tablet on the same wifi network then use the developer views in Safari and Chrome to see how your site might appear.
Many of us long time Sparkle users will have template project set up. In my case, this consists of a page with all my text styles defined at sensible point sizes for each device. Similarly, buttons, menus, etc. Careful thought about readability, etc. before you even drop an object on the Sparkle canvas will be well rewarded. Pen(cil) and paper still have a role in design!
I absolutely concur with @duncan
If you find you are having to clear the Show on this device checkbox to restrict content then you should be asking why!
So, design for users first with good, relevant content displayed well on all devices and you are home and dry!