If you click on the icon that appears in the middle of your left-hand screen, a menu opens in which you can perform the following actions;

Increase and decrease the font size
Grayscale, all content will become grey at different scales differentiating content
High contrast, the contents move to a high contrast mode, to differentiate the contents
Negative contrast, all content becomes a negative contrast helping to differentiate sections
Clear background transforms the entire web and puts a white background to it.
highlight links, highlights all links available on the web
Legible source, transforms site sources to a legible source
Reset, reset all default values of the web.