Update readme_banner.svg to support dark mode

[vinorodrigues]

readme_banner.svg is not visible on github.com when a browser is in dark mode. (Black path > fill on dark background. BTW: CSS color currentColor - not set in original SVG, but assumed by browsers, does not propagate in embedded SVG images, i.e. SVG does not take on color of hosting HTML page.)

This PR adds a <style> tag to the SVG to accommodate darkmode in the browser - allowing the image to be seen when github.com is viewed by a browser in that state.

(Based on my work done on bootstrap-dark circa. Dec. 2020)

i.e. added the compressed following to the SVG

<style>
  path{
    fill: #111;
  }
 @media( prefers-color-scheme: dark ) {
    path{
      fill: #EEE;
    }
  }
</style>

[ashmaroli]

LGTM! 👍

[ashmaroli]

Thank you very much, @vinorodrigues :)

@jekyllbot: merge +doc