Custom dialogs not updating theme correctly

[Jacalz]

Describe the bug:

Changing theme when an application is running does not take effect correctly on custom dialogs and causes elements of the screen to behave weirdly.

To Reproduce:

Steps to reproduce the behaviour:

1. Change theme of the currently running app using app settings API.
2. Open a dialog with some custom stuff in it.

Screenshots:

Device (please complete the following information):

• OS: Linux
• Version: 5.6.18
• Go version: 1.14.5
• Fyne version: 1.3.2 and latest develop ea960bd

[andydotxyz]

I have a fix locally - the issue is that the background colour is not smart and nothing in the container was updating it on refresh.
Gonna see if I can clean it up a little in the morning :)