Magento-2 migration involves four components:
1. data
2. extensions (or) Custom module
3. themes
4. customizations
1. Data:
This data migration tool will help to move products, customers order details , store
configuration details, promotions and more values from Magento 1.x to Magento2.x.
Only default tables values we can move from 1.x to 2.x. In case if we create any custom
table or if we create a custom column from magento tables (Ex: sales_flat_order) will not
able to move as of now. (Data Migration tool won’t support with current available
version)
2. Extensions or Custom module :
Migrating Extenstion from M1.x to M2.x is in-progress by Magento team. Magento team
is still developing the new module (or extenstion) which help to move the extentions
which is used from M1.x to M2.x.
3. Themes and Customizations:
As of now there is no tool for migrate theme which is used on M1.x to M2.x.
Reason :
(3.1) M2 theme & design template & Css & Js folder structure is totally differ from M1.x.
(3.2) We can’t simply copy paste and use the layout and template files directly into M2.x
(3.3) Finally The answer is – we can’t migrate themes in Magento2.x