Remove Panel Border/Background

Hello,

I can't figure out how to modify the theming to remove the white border on top and below of the ggupset panel. This is really only apparent when theming the whole plot to be a dark theme (see image where I modified the strip.background.colors). Any theming I do doesn't seem to touch the ggupset panel, although it does the geom_bar().

```
ggupset::scale_x_upset() +
ggupset::theme_combmatrix(combmatrix.panel.striped_background.color.one = "gray20",
combmatrix.panel.striped_background.color.two = "gray30")
```


Looking at the ggplot object `$theme` doesn't give any hints as well...

```
$ combmatrix.label.make_space : logi TRUE
$ combmatrix.label.extra_spacing : num 3
$ combmatrix.label.total_extra_spacing : 'simpleUnit' num 10points
..- attr(*, "unit")= int 8
$ combmatrix.panel.margin : 'simpleUnit' num [1:2] 1.5points 1.5points
..- attr(*, "unit")= int 8
$ combmatrix.panel.striped_background : logi TRUE
$ combmatrix.panel.striped_background.color.one: chr "gray20"
[list output truncated]
- attr(*, "class")= chr [1:2] "theme" "gg"
- attr(*, "complete")= logi TRUE
- attr(*, "validate")= logi FALSE
```

Can you point me to where in the code this white box/border may be coming from?

Thanks,

Jeff

Reproduce the dark-theme example using ggupset::scale_x_upset() and theme_combmatrix(), then inspect how the ggupset panel is rendered and themed. The issue is done when the white border/background can be removed or themed consistently with the surrounding ggplot panel.

Written by the indexing model from the issue text.