Inconsistent style.css references in Theme Directory strings

[SergeyBiryukov]

• In some instances, <code>style.css</code> is moved out of translatable string (good)
• In most instances, <code>style.css</code> is a part of translatable string (bad)
• In some instances, style.css is a part of the string, but without <code> tags (meh)

Let's make it consistent and add translator comments for placeholders.

[SergeyBiryukov]

Duplicate of #1058.