Accessibility - colours and theme

### Current Behavior

Web user interface is a "dark mode" - dark background, with light text.

### Proposed Behavior

Ability to choose between "dark mode" and "light mode" colour themes.

There's a growing preference for "dark mode" themes in applications, and sometimes even a mistaken impression that they are universally better for accessibility. This is not the case for all users. I find "dark mode" themes very uncomfortable, to the extent that it becomes an accessibility issue.

### Checklist

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

No files, tests, or entry points are named. Start by locating how the frontend currently defines and applies its dark colour theme, then determine how a user would select light or dark; done means both themes are selectable and the light option addresses the reported accessibility concern.

Written by the indexing model from the issue text.