Update the UI theme

[Senyoret1]

Did you run make format && make check?
The go code was not changed. npm run lint and npm run build were used.

Fixes #260

Changes:

• This PR changes the theme of the Skywire manager, to make it look like the one in the issue.

How to test this PR:
Check the changes in the Skywire manager.

NOTE: at this point most of the pages have the dark theme, except for the options page. The modal windows have not been changed, so the design still needs improvements.

[Senyoret1]

The last commit includes the following changes:

• Now the options page of the manager uses the dark theme.
• Generic styles for making the fields and buttons work well with the dark theme were added.

[Senyoret1]

The last commit includes the following changes:

• The design of the modal windows was changed to work better with the dark theme.
• Visual improvements were added to several modal windows, appart from the ones applied to the generic modal window component.
• The design of the modal window for updating one or all visors at the same time has a lot of changes and some potential problems in edge cases were fixed.
• A bug with the theme background was solved.
• A bug with the login page was fixed.
• Some other small fixes.

[Senyoret1]

The last commits include the following changes:

• SelectOptionComponent was improved and now if replaces several other modal windows, to simplify the code and ensure a more consistent design.
• An inconsistency with the selection of the default transport, in the transport creation modal window, was solved.
• A problem with modal windows without scroll bars was solved.
• A problem with the layout just after initiating navigation from the visor details page was solved.
• Some other small improvements.

[Senyoret1]

The last commits include the following changes:

• The problem with the Travis test was fixed.
• The way in which the scss imports are made was changed, to avoid repeated styles.

[jdknives]

@Senyoret1 I sent some requests/ideas on TG.

[Senyoret1]

The last commit includes the following changes:

• Now the background is darker.
• Now the flag button has a white background.
• Now the menu does not have a clear gradient at the borders.