Accessibility - sufficient color contrast against the background

### Current Behavior

Text (Date in the graphs, Graph subheadings, Titles of the portfolio statistics) in the dashboard page has poor colour contrast.
WCAG requires a minimum ratio of 4.5:1 for small text.

### Proposed Behavior

Contrast text in Text (Date in the graphs, Graph subheadings, Titles of the portfolio statistics) with the background should be 4.5.1 minimum

### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/main/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/frontend/issues) for whether this enhancement was already requested

Start at the dashboard page and locate the graph dates, graph subheadings, and portfolio-statistics titles named in the issue. Check each text/background pair with a WCAG contrast checker; done means the affected text reaches at least a 4.5:1 contrast ratio.

Written by the indexing model from the issue text.