frontend: use darker text color on charts

[jakobhellermann]

fixes #2215

Dark:


Light:

[jakobhellermann]

as is, this messes up the date texts

[yagebu]

Thanks for opening a PR. I think the fix here needs to be to have different colors for the different texts:

• the legend is on the background, which is different in dark and light mode - so these texts also need to adjust
• the chart colors are the same in dark and light, so the text color there should probably stay the same as well

[jakobhellermann]

Okay I changed the PR to only apply the new text-on-chart color for the icicle and treemap charts, and keep the rest as is.