General Support: Re: W3C ??
I have added "en" to the html tag (allowing users to specify language is a good idea, but in practice they're generally English on Voice). I've also added a label to the search input for screen readers.
Regarding the themes - yes you're right the contrast isn't great on the some of the menu links etc. You could try the alternative menu bar colour or different colour schemes. I'm afraid I don't know offhand which ones have the most contrast.
The colour schemes come from BootSwatch. You can look at the samples on their site and decide which you like before trying them on your site. (note that we use Bootstrap v3 not v4)