Horizontal bar graphs' bars disappear in dark mode

[anambcodes]

Describe the bug
When the Dark Mode extension is activated in Chrome, the bars in the horizontal bar graphs in Insights visually disappear.

To Reproduce

• download the Dark Mode Google Chrome extension
• activate the extension (Current State: Dark)
• go to Insights and scroll down to graphs

Expected behaviour
The bars would appear in a suitably contrasted colour, e.g. the 360Giving yellow (or the orange used in the award date bar graph, though that's slightly less contrasted but still sufficient for the minimum WCAG requirement).

Screenshots: