It's for document. Every document can have your preamble. I store my favorite preamble on the the default model of libreOffice and at starts it is there.

\usepackage[brazil]{babel}
\usepackage{amsmath}
\usepackage{sfmath}
\usepackage{DejaVu Sans Condensed}
\renewcommand*\familydefault{\sfdefault}
\usepackage{cases}
\usepackage[T1]{fontenc}
\usepackage{amssymb}
\usepackage{mathastext}
\usepackage[utf8]{inputenc}
\usepackage{sansmathaccent}
\usepackage{mathrsfs}
\usepackage[normalem]{ulem}
\usepackage{graphicx}
\usepackage{xcolor}
\definecolor{navy}{HTML}{000080}
\definecolor{mogg}{HTML}{000066}
\definecolor{plug}{HTML}{800000}
\usepackage[usenames]{color}

% Uncomment this line for sans-serif font
\everymath{\mathsf{\xdef\mysf{\mathgroup\the\mathgroup\relax}}\mysf}

% Uncomment these lines for colored equations
% Caution! Background color breaks transparency!
%\definecolor{fgcolor}{RGB}{0,0,102}
%\definecolor{bgcolor}{RGB}{255,255,255}
%\pagecolor{bgcolor}\color{fgcolor}

You can save your blank document like default on the models, save as models, my models and mark save as default.