Description
This is a revdep issue (#3303) related to #3310 in that the theme is the thing that caused the plot object to change from the previous version, but different in that the downstream problem is how to properly test ggplot output.
MTLR does not define a complete theme, but has a .rds file containing a previous theme object that does not match the current ggplot theme objects (since they have all changed).
ggstatsplot tests its custom theme_pie() using numeric indices ( https://github.com/IndrajeetPatil/ggstatsplot/blob/master/tests/testthat/test_theme_ggstatsplot.R#L65 ). The custom theme is created using theme_bw() + theme(...) and is probably fine, but the index of the object they're trying to test has changed since an element was inserted.
CSTools has a test for the length of the theme object ( https://github.com/cran/CSTools/blob/master/tests/testthat/test-PlotForecastPDF.R#L13 ).