Adding dark mode

[voruti]

Closes #14

Btw: The nodejs.yml workflow seems to be broken ...

[voruti]

The missing issues of the "Codacy Static Code Analysis"-check are senseless:

1. "selector-type-no-unknown": Okay, fine, but they are correct. Maybe add them to a list of "known" tags?
2. "no-descending-specificity": These have no influence on the resulting style, but I ordered them like this to keep them in categories (e.g. chat selection, chat history, settings, profile, etc.). Maybe ignore these checks?

What to do now?

[voruti]

Btw: The nodejs.yml workflow seems to be broken ...

Fixed in #16

[callFEELD]

Heya, thank you for your work so far. Appreciate it!

I also partly worked on dark mode (didn't push it). But since some people prefer dark mode and some light mode I would suggest adding an option to switch between both designs.

But I am not sure how the settings options should be integrated tbh.

Maybe you've got an idea.

[callFEELD]

Quick review point for the CSS:

Could you use CSS variables for the color values. Makes changing the color values easier.

[voruti]

But I am not sure how the settings options should be integrated tbh.

I also thought about that and simplest solution would be to adopt the operating system settings (that's how I already implemented it - these CSS rules are only activated if the OS is set to dark mode).

Another idea I had, was to add a new context menu item to the tray icon, which would toggle the dark mode on and off.
This gives rise to some questions: What's the default value? How to save this setting/toggle across app restarts? How to implement this into/with electron? (I never worked with electron before) ...

[callFEELD]

The missing issues of the "Codacy Static Code Analysis"-check are senseless:

1. "selector-type-no-unknown": Okay, fine, but they are correct. Maybe add them to a list of "known" tags?

2. "no-descending-specificity": These have no influence on the resulting style, but I ordered them like this to keep them in categories (e.g. chat selection, chat history, settings, profile, etc.). Maybe ignore these checks?

What to do now?

Removed the senseless checks: https://app.codacy.com/gh/callFEELD/Threema-Desktop/pullRequest?prid=7565215

Next commits will probably pass the test.

[callFEELD]

Everything looks good!