Switching between dark and light theme only works once

[mtausig]

Description

If I open the settings in the riot desktop client, I can switch between the dark and the light theme.
If I change this option once, the theme immediately changes, but changing the option again, doesn't do anything. To change the theme back, I have to close the settings dialog and reopen it.

Steps to reproduce

1. Open Settings
2. Click on "Dark theme" in the USER INTERFACE section
3. Click on "Light theme" in the USER INTERFACE section
4. Close Settings
5. Open Settings
6. Click on "Light theme" in the USER INTERFACE section

After 2., the app goes dark, but it doesn't change back in step 3.
Step 6. finally returns me to the light theme

Version information

riot-web-0.12.2 running on Ubuntu Xenial 16.04 installed from the official APT repo at https://riot.im/packages/debian

[t3chguy]

Duplicate of #4700

[mtausig]

Sorry.