Inkscape is professional, free, open source Vector Graphics software.

Notes from this tutorial:
To change the thickness of a stroke, click on Object in the menu bar and select fill a stroke to bring the fill and stroke options. You can also right-click the object and select the option there.
The Fill and Stroke Options have three tabs:
The Fill Tab allows us to change the color of the object body using the RGB, HSL, CMYK, Wheel, or CMS options.
In the Fill tab, next to the X there are also available options to apply linear, radial, and mesh gradient, as well as pattern and swatch.
When the Pattern option is selected you can choose from many Pattern fills, or apply Blur, and Opacity effects
The Stroke Paint Tab allows us to change the color of the Stroke with the same Flat color options (RGB, Wheel..).
Use the Stroke Style to change the Width and other characteristics of the stroke.

Special thanks to @rvalle13 for providing these notes!

You can download it today, on Windows/Linux/Mac at:
https://inkscape.org/en/