$ vim ~/.config/sublime-text-3/Packages/User/Default.sublime-theme
 
[
    ...
    {
        "class": "sidebar_label",
        "color": [0, 0, 0],
        "font.bold": false,
       	"font.size": 15,
       	"font.face": "Open Sans"
    },
    {
        "class": "tab_label",
        "font.size": 15, 
        "font.face": "Arial"
    }
]