。
The world seen through the lace curtains was dark.
Literal
Lace [possessive-の] curtain [from-から] showing-through seen world [topic-は] dark was.
透けて見えた ('showed through and was visible') is a compound: 透ける 'to be transparent/show through' + 見える 'to be visible.' The layered viewing — world seen through semi-transparent fabric — creates a literary, atmospheric image. The darkness beyond the curtain has an ominous or melancholy quality.