Fix illegible white on yellow text of stacktrace

[NikolayXHD]

Higlighted stacktrace background color is inherited from default theme, which is bright yellow,
while default text foreground becomes also bright in dark themes, thus becoming unreadable.

before

after

For reference, there is analogous PR to default JupyterLab dark theme

[dhirschfeld]

It would be great to get this in! This theme is great, but this is a pretty bit pain-point with day-to-day use.