Refactor how themes are provided. Fix a bug with tab switching.

[mddragnev]

Closes #21

[skrustev]

These are the few issues i've found:

1. The height of the grids in each tabs is different. Event if its not 100%, at least should be consistent.

2. The icon in the tabs download button is now not visible, except for the currently selected one:
   

3. The theme for all grids is not correctly rendered. The root material theme leaks onto the other grid themes:
   For example how it is for the HR Portal:
   
   How should be(except the trial marks ofc):
   

[sbayreva]

1. There should be Top, Bottom, Left and Right paddings of 16px around the container that holds all the tabs. Now there is a padding between the tabs container and the grid, but it is 24px which is not according to the design.
   Implementation:
   

Design:

2. Тhe background color of the tabs container should be surface while (Material)

3. In the ERP/Inventory grid, I can still see stretched images. Not all of them but some of them:

5. Fleet Management. The image here does not fit right. Maybe the height should take 100%
   

[mddragnev]

@sbayreva,
For 3. you can pull latest and look at the images. They should be ok.
For others (1,2,4) could you please log them as a separate issues since they are not part of the current effort.