How do i bring up the setting for the webnovel.com on my phone and change the layout like in the desktop version?
You can't. That's just one of many bugs, e.g., the latest chapters aren't always indexed or the front page doesn't have a mobile version. You can write to them suggestion(at)qidian(dot)com, maybe they'll respond. Regarding colors you could use a browser with an inversion feature, but I don't know what apps there are for iphones.
The site is still beta for phones and tablets so you will have to wait until they release an upgraded version
Your phone has a specific ip which are for mobiles only, if you're using an android it is possible to trick the site into thinking you're using a desktop. This doesn't work on iOS.