What contrast ratios should a Bootstrap theme target?
For ordinary text, WCAG AA generally requires a contrast ratio of at least 4.5:1. Large text can pass AA at 3:1. AAA raises those targets to 7:1 for normal text and 4.5:1 for large text.
Use the checker for buttons, links, alerts, badges and any custom Bootstrap color pair. The main Theme Studio can also choose black or white foreground text automatically based on the better contrast ratio.