You'll need to open the command palette (ctrl-shift-p) and edit your settings file. Add this to the themes:
{
"background": "#282828",
"black": "#1D2021",
"blue": "#458588",
"brightBlack": "#928374",
"brightBlue": "#83A598",
"brightCyan": "#8EC97C",
"brightGreen": "#B8BB26",
"brightPurple": "#D3869B",
"brightRed": "#FB4934",
"brightWhite": "#FBF1C7",
"brightYellow": "#FABD2F",
"cursorColor": "#FFFFFF",
"cyan": "#689D6A",
"foreground": "#EBDBB2",
"green": "#98971A",
"name": "Gruvbox?",
"purple": "#B16286",
"red": "#CC241D",
"selectionBackground": "#FFFFFF",
"white": "#EBDBB2",
"yellow": "#D79921"
}