Allow CSS theming of comment border colours

[weiteck]

Requirements

• [X] This is a feature request and not a bug report. Otherwise, please create a new bug report instead.
• [X] Please check to see if this request (or a similar one) already exists.
• [X] It's a single feature. Please don't request multiple features in one issue.

Describe the feature you'd like

Currently comment border colours, indicating the depth of a comment within the comment tree, is injected in the HTML <style> tag with an !important override and there is seemingly no way to change the colours with a CSS theme (please correct me if I'm wrong). I propose that a new CSS class be added to comments that corresponds to its depth in the comment tree, e.g. comment-level-0, comment-level-1, etc. where 0 would be a top-level comment, 1 would be a level below that, and so on. The styling would be implemented in the external CSS files, allowing it to be overridden with a CSS theme file.

This could be taken a step further with an additional class added, either comment-level-odd or comment-level-even, so a theme could just easily use alternating colours instead of unique colours for each level in the tree.