A restrained theme: left-aligned titles, recessive grid, no panel border, and small uppercase-free strip labels. All plotting methods in the package use it; add it to your own plots to match.
Examples
if (requireNamespace("ggplot2", quietly = TRUE)) {
ggplot2::ggplot(mtcars, ggplot2::aes(wt, mpg)) +
ggplot2::geom_point() +
theme_fmridesign()
}