solarized: more consistent and subtle md / org modes

### What did you expect to happen?

Headers and bold text are similarly shown in org and in markdown. Also the highlighting is more or less subtle. Besides it's consistent between light and dark modes.

### What actually happened?

org and markdown modes are inconsistent in their coloring of similar semantics. Moreover, in markdown headings and bold text are highlighted using a very distracting red (usually destined to errors). Besides, in solarized light bold and heading are highlighted almost the same, but not in solarized dark.



### Describe your attempts to resolve the issue

I did nothing yet, but I believe the best approach is to use the current colors for org mode also in markdown mode.

### Steps to reproduce

1. Set theme to solarized light.
2. Create a markdown file with heading and bold text
3. Create an org file with heading and bold text

### Package commit

e716ddbb882a3a06744faa74decb2fea1569c921

### System Information

macOS Big Sur, emacs-28 branch.

Reproduce the Solarized light and dark cases with matching headings and bold text in Markdown and Org modes, then inspect the theme face definitions and the mode-specific faces they use. Compare the corresponding colors and adjust the theme so equivalent semantics are consistent and subtle across both modes and variants; verify by repeating the listed steps.

Written by the indexing model from the issue text.