Apply user's theme to the link tooltip

[mickmister]

Currently the link tooltip is styled with a white background etc. It should instead match the user's theme to blend well with the center channel.

Issue created from a Mattermost message by @gabrieljackson.

[hanzei]

@fedealconada Would you be interested in working on this given that you introduced the feature?

[fedealconada]

Sure @hanzei !

[fedealconada]

Hey @mickmister, I've linked the user's theme to the tooltip but there are some styles that are not passed in the theme's... I think that the things most affected are:

1. Box shadow of the tooltip
2. The little boxes specifying the repo branches (above See more link).

So, a couple of alternatives:
Option 1. We leave it like that (its not bad though).
Option 2. We add the Menu__content class to it, which will add some borders.
Option 3. We could add some additional classes to the theme but they could be a bit specific to the github tooltip

Option 1

On the following image, note the Repo <- Repo in which there's not enough contrast between the light blues

On the next two images, note the lack of box shadow (maybe not a big deal)

Option 2
On these images, I've added the Menu__content class. Note the rounded corners and the border color.

[hanzei]

@asaadmahmood What do you think about these ideas ☝️

[asaadmahmood]

@fedealconada We can keep the box shadow black.
However, the border color of the tooltip can be: center-channel-color-16.
Also, can you increase the z-index of the tooltips to be 1070?

[fedealconada]

hey @asaadmahmood, I've just added that. It would look like this:

Thoughts?

[asaadmahmood]

@fedealconada Looks great to me, one thing though, add a border-radius: 4px to it as well.