Value label on top of traffic graph bars difficult to read in dark mode

## Expected Behavior

All traffic graph text should be easily readable in light and dark mode.

## Current Behavior

When using dark mode, the value label on top of traffic graph bars-- meaning the value can't be displayed within the bar-- is difficult to read.

This happens with any instance of the traffic graph (`/system/overview`, `/system/enterprise`, `/system/licenses`), but does _not_ happen when doing an aggregation in a search (e.g. `sum` of `gl2_accounted_message_size`).

Traffic graph:

Aggregation:

## Steps to Reproduce (for bugs)

1. Be on `6.3.5` or `7.0.0` (maybe newer) and use dark mode.
2. Look at a traffic graph where a bar's value label can't "fit" within the bar.

## Context

Just a minor inconvenience, as the info can easily be seen by hovering over the bar.

## Your Environment

* Graylog Version: `6.3.5` and `7.0.0`
* Browser version: Firefox `144`

## Checklist
[] This issue fix need to be backported.
[] Does this issue have **security** implications?