Most web browsers allow the user to zoom/enlarge the webspage and font size, and override stylesheets to choose your own colours to make things easier to view:

a) In Firefox: Click tools > options > content. Under “fonts and colours” click colours and choose the best colour combination for your use (yellow on black is used a lot, or in this case you can change to black on white). Make sure you uncheck “allow pages to choose their own colours”. This will override any styles on any websites.

b) In Internet Explorer: click tools > internet options > general and click the “accessibility” button. Check the “ignore colours specified on websites” button. This will default to black text on a white background (with blue links) although you can click the “colours” button on internet options and choose the colours you want to use.

c) Google Chrome also has a number of accessibility options – you can see a guide here  or search for ‘Google Chrome Low Vision Support’