Every section chooses one of the five color schemes in its own settings, at the top, under Color scheme. If one section looks wrong and the rest look right, it is using a different scheme from its neighbors: open the section and pick the scheme you want.
If every section using a scheme looks wrong, edit the scheme itself under Theme settings → Colors. Each scheme has Background, Surface, Text, Secondary text, Borders and lines, Buttons and accents, Text on buttons and Links. Keep text at 4.5:1 contrast against its background.
Hero slides have their own Text color scheme, which sets the text and the overlay tint on that slide. The full picture is in Colors and color schemes.