What does "steal" in this Oscar Wilde piece mean

Solution 1:

There are several meanings for steal as a verb. Either of these might apply

to move or convey stealthily - they stole along the corridor

to pass unnoticed - the hours stole by

Wilde could have been suggesting that his hair would fade little by little, almost unnoticed. Even more likely, he could be suggesting that the color of his hair would sneak away. Just as the color of his lips is the actor in the previous clause, the color of his hair is doing the moving in the clause in question.

Solution 2:

To "steal from" means to leave. It often is suffixed with away or off, but this is not necessary. For example, the OED quotes:

1639 T. Fuller Hist. Holy Warre iii. xvi. 135 Other Captains secretly stole home. 1786 F. Burney Diary 18 July (1842) III. 22 The sub-governess, stole from her charges, and came to the window. 1869 H. F. Tozer Res. Highlands of Turkey II. 267 Maria stole off to the honey.

I think that, poetically, the association with robbing gold is also intended.

Solution 3:

The "away" from "pass away" also belongs to the "steal". So it is "steal away".

gold steal (away) from his hair means that not only his lips will bleaker, but also the color will fade from his hair, when he gets older.