I am not sure how you can do that with rstan’s functions but this work for cmdstan output and traceplots:
# install.packages("remotes")
remotes::install_github("stan-dev/cmdstanr")
library(cmdstanr)
library(bayesplot)
fit <- as_cmdstan_fit(files)
color_scheme_set("mix-blue-pink")
mcmc_trace(fit$draws(variables = c("alpha", "beta[1]")))
