Display bug in bc.pl.gene_expr_split_stacked

Split is not so clear. Colors are grey ?

```

adata = bc.datasets.Haber2017_processed()

bc.pl.gene_expr_split_stacked(adata=adata, genes=['Defa24', 'Gm15284'], split_variable='donor', subset_variable = 'region')

```


Start by locating the implementation of bc.pl.gene_expr_split_stacked and reproduce the supplied Haber2017_processed example with genes Defa24 and Gm15284, split by donor and subset by region. Compare the rendered output with the attached screenshot and establish the intended color and split presentation before defining the fix and its validation.

Written by the indexing model from the issue text.