Thank you! For the highlighted plurk responses, I used

.plurk_cnt.new1, .plurk_cnt.new2, .plurk_cnt.new3, .plurk_cnt.new4, .plurk_cnt.new5, .plurk_cnt.new6, .shade_1, .shade_2, .shade_3, .shade_4, .shade_5, .shade_6 {
background: #001128;
}

which is basically just a combination of the solution in this post and the solution offered by hypeodermic--neither worked fully for me either, but combined they seem to do the trick more often than not.

The various text entry areas are a combination of a couple of elements. Since I'm using the same color and fonts for both text boxes, first I have this bit, which defines the actual text entry area:

.textarea_holder textarea {
background: #76A1B5;
opacity: .5;
font-family: 'Gill Sans','Futura','Verdana',sans-serif;
}

Then I have the area immediately around the text fields, which is basically like the margins of the paper when you're editing a Word-style document:

._lc_ .input_holder {
border: #eee 1px solid;
border-color: #000;
border-radius: 8px;
background: #060F2B;
}

The opacity setting in the first section lets you play with the shade of the text boxes without having to mess with different hex codes, but if you have a specific color in mind you can omit that line or change the value to 1.0 to keep the input_holder color from affecting the textarea color.

If you want the reply boxes to be a different color from the new plurk text box, you can use the following respectively (the latter of which I see is already in your code:

.mini_form .textarea_holder textarea {
background: #xxxxxx;
}

.input_holder #input_big {
background: #xxxxxx;
}

Does that answer your questions?