The "Welcome" screen wrongly keeps a Dark background after the theme is changed to Light/Default

[Softvision-CristinaBadescu]

[Affected versions]:

• Firefox 60 and up
• Side View 0.4.3

[Affected Platforms]:

• All Windows
• All Mac
• All Linux

[Prerequisites]:

• Have the Dark Theme enabled.

[Steps to reproduce]:

1. Open the Firefox browser with the profile from prerequisites and navigate to the "about:addons" page-> Extensions tab.
2. Install the "Side View" experiment.
3. Change the browser theme to the "Default" one ("Open menu" button -> Customize).
4. Observe the "Side View" sidebar.

[Expected result]:

• The sidebar background changes from dark to light.

[Actual result]:

• The background remains dark.

[Notes]:

• This issue is also reproducible for other light themes.
• This issue is no longer reproducible if you switch or close the sidebar.
• As mentioned in #239 PR, this issue can be addressed after bug #1435216 is fixed.
• Here is a screen recording with this issue:

[ghost]

See bugzilla bug linked above