I would suggest that the theme of order and chaos is embedded in Hardy's poems through a couple of ways. One such way is in the structure of the poem. The poem's structure is in a conventional meter and rhyme. The form of the poem helps to provide some level of order to a thematic exploration where there is only chaos and confusion. There is little in way of certainty of what the speaker, presumably Hardy, will find in the future. The poetic construction of the poem is the order through which Hardy is able to explore the chaos that is a part of the modern setting.
The theme of chaos and order is also seen in the song of the thrush, itself. It can be interpreted as an example of the theme in a few manners. The speaker does not know why the brush sings the evening song it sings. There is a beauty evident in this moment that the speaker captures. Yet, why it does what it does cannot be fully explained. The dissonance this creates helps to establish the order and chaos theme. Another way of examining the theme in this moment would be in the reveling of the moment, itself. In the midst of a world that has lost its connection to a traditional past, something about which the speaker feels terrible, the song of the bird reminds the speaker of the order that does in fact exist. There can be beauty. There can be redemption even in a setting that might appear devoid of it. In this, there is an establishment of order in a world of chaos.
Finally, I think that the exposition to the poem might also support the chaos and order theme. The speaker leans on a gate. The speaker examines the world and all of its implications from a place of order and structure. While there is a clear indication of chaos all around the speaker, the ability to lean on the gate and find support in a world where it is absent is part of the order and chaos thematic dynamic that is a part of the poem.