Enable light mode or theme option on new website

[lazer15]

When the print tuning guide was available on github there was an option to force light mode using the github settings in order to improve readability for people who sometimes have problems viewing sites with a dark color scheme. This is no longer possible now that the print guide has been removed from github and the new website forces a dark theme with no option for a light theme.

[AndrewEllis93]

Unfortunately this causes issues with some of my images that use transparency, some colors are hard to read on white background, etc - I will put this as a "nice to have" for now