It seems worth providing a simple answer about the basics of SVGs to go alongside the details provided here about specifics around software...
An SVG file is just a text file. The file ending will be .svg but it can be opened in a text editor the same as a file with the ending .txt
A simple svg file looks something like this:
<svg width="580" height="400" xmlns="http://www.w3.org/2000/svg">
<!-- This is a comment -->
<rect id="rectangle1" height="29" width="27" y="50" x="57" stroke-width="1.5" stroke="#000" fill="#ff0"/>
<ellipse ry="17" rx="16" id="circle1" cy="64" cx="113.5" stroke-width="1.5" stroke="#999" fill="#aaa"/>
<path id="path1" d="m56.5,97l51.5,1l5,25l-29,-5l-27.5,-21z" stroke-width="1.5" stroke="#f00" fill="#fff"/>
In this you can see that the colours are defined by the parameters (where the dots are replaced with characters between 0-9 or A-F):
And the stroke width is defined by
The following can be added, using the text editor, to replace whatever defined values you choose in the SVG... making it possible for the values to be set by QGIS. QGIS understands a value for 'param(fill)' - and the other 'param(...)' values - as these are set by QGIS itself.
Outline / line colour:
Outline / line width:
Outline / line opacity:
Take note that there's nothing to stop you using the 'param(...)' values in slightly odd places - so for instance you could use the 'param(fill-opacity)' value to define a fill colour in one bit of the SVG file, but a stroke/line colour in another bit of the SVG.
In the QGIS composer or style symbol dialogue the settings for the SVG colours are fairly obvious. The fill and stroke/line opacity is set as part of the colour settings. The stroke/line width setting is obvious.
Lastly two points
First, note that while in an SVG it is valid to define the colours as a collection of attributes like so:
This appears not to work properly if you are including parameters as described here.
Second, when using software like Illustrator or Inkscape lots of additional attributes can be added to the SVG text by this software. If you know what you're doing and understand the basics of an SVG as above it may make life simpler if you get rid of this additional text. In particular you may find that colours are set through the definition of styles rather than item by item.