OK, I think the easiest changes for you would be, in the editor, change every instance of
Code:
background-color: rgb(0, 0, 153);
to
Code:
background-color: white;
and every instance of
Code:
color: rgb(153, 255, 255); or
color: rgb(204, 255, 255);
to
Then save the HTML file. If you double-click on it, it should open in your browser showing black text on a white background throughout.
Once you're happy, reimport the HTML file into calibre and do your conversion again. The bright colours should be gone.