Can't read "Allocated" and "Total" in light mode

### Description

When inspecting the default "OpenStack Compute Overview" dashboard in light mode it is not easy to read the values in the "Allocated" and "Total" panels. There could possibly be other areas, but these are the two I noticed.

Inspecting the panel its possible to change the values, but here is the problematic default.

```
"steps": [
{
"color": "rgb(251, 252, 250)",
"value": null
}
```

Looks like this is defined in [grafana_dashboards/cloud.json](https://github.com/canonical/openstack-exporter-operator/blob/main/src/grafana_dashboards/cloud.json) and [grafana_dashboards/compute.json](https://github.com/canonical/openstack-exporter-operator/blob/main/src/grafana_dashboards/compute.json).

### Charm revision

Revision 58 on latest/stable

### Juju version

3.5.7

### Cloud

OpenStack

### Expected behaviour

Visible text that is easy to read

### Reproduce / Test

Deploy the openstack-exporter charm and change to the light mode view for grafana

### Notes & References

_No response_