Have you applied the colors, fonts and font sizes to your Theme Manager yet? If you have a dark site, the theme manager should be set to use a lighter font. When you drop in a new text block, you simply need to apply the correct selector and if your theme manager has that select as a white font, it will stay a white font.
Not sure what would change between editing and post-publishing. Can you share a link?
You can control your stack to be FIXED or FLUID. I like mine set to utilize FLUID but with Max/Min widths. Select stack and open inspected. Size it like you size all elements.