Can't move widget width

In your case, it’s better to use the col span feature rather than modifying CSS.

You have three columns in one row, which is the default setting.

Now set

1-3

in the Column Spans option. This makes the first item have three column spans.

Hope this helps.

Since your body tag has min-width: 900px, maybe this is what you want,

.responsive_column_widgets { max-width: 900px; }

If not, I need to get more precise information what you want to achieve.

For instance,

The ‘Label’ section refuses to budge (I would ideally like the whole thing to be centered under the thumbnails).

I need to know what the whole thing refers to.

Also you would need to remove previously added custom CSS rules for the plugin if you have not.

I’d like Label, archives, and the search to sit center underneath the thumnails on the front page.

I really don’t get what you are trying to achieve. If you don’t need columns, just set 1 for the Columns option.

Also, would I have to remove all CSS, even ones styling the text?

It is recommended to start from the beginning as you seem to have messed up the style of the plugin. To be clear, remove the CSS rules you added to modify the plugin’s layout, not the theme’s.

I see.

Maybe this works

.responsive_column_widgets .responsive_column_widgets_box { max-width: 900px }
.responsive_column_widgets_column { text-align: center; }

or

.responsive_column_widgets .responsive_column_widgets_box { max-width: 900px }
.responsive_column_widgets_column {
	margin-left: auto;
	margin-right: auto;
}

You’re welcome!