EU Cookie Law widget doesn’t generate inline display style

I have an odd issue in that if I use a very basic theme, the EU Cookie Law widget doesn’t display, yet it does when I switch to an in-built theme such as Twenty Fifteen.

The weird part is that with my basic theme it does generate the HTML code, but it doesn’t generate the inline style which displays it.

Example for my basic theme:
<div id="eu_cookie_law_widget-4" class="widget widget_eu_cookie_law_widget">

Example for the same page in Twenty Fifteen:
<aside id="eu_cookie_law_widget-4" class="widget widget_eu_cookie_law_widget" style="display: block;">

You’ll see that for Twenty Fifteen, it also adds an inline style which causes it to display. If I use the dev tools in Chrome to add the style to my active page with the basic theme, it displays.

Why does the inline style not get generated in my basic theme, and how do I fix it?

(my basic theme is one I’ve written, and I have no knowledge of how these things work, it’s just cut and pasted from the interwebs)

I’ve tried using the “Widget Visibility” feature of Jetpack so that it Shows on Post type: Pages (so it displays on all Pages, which is all that’s on the website), but it didn’t change it, so I deactivated the Widget Visibility feature, and it still didn’t display, even after I’d deleted the widget and started again.

Any help appreciated.