Font size and spacing tweaks

A possible fix for #59. Two differences which I'll point out are that there is now more white space above post titles, and the heading sizes on mobile-sized screens have changed a bit. 

Have a go with it though, and see if you like the changes.
Either is valid: with 3 values, they correspond to `top horizontal bottom`. (From [MDN](https://developer.mozilla.org/en-US/docs/Web/CSS/margin))