I fixed the main theme setup page (moodle/theme/index.php).
1. Fixed styles on lines 112, 118, 120 (changed "font-style: bold" to "font-weight: bold", fixed "border-color=#555555")
2. Fixed code on lines 125-130. If there were no "$readme" AND "$screenshot" the code generated empty <ul></ul> element. Just moved "echo '<ul>'" and "echo '<ul>'" inside the "if ($screenshot or $readme)". Now page validates.
I hope it helps.
Thanks for your posting. However, I thought that since the introduction of the new themes system in Moodle 1.5 no hard-coded formatting should subsist anywhere in any of Moodle's php files. I have posted here about this problem.