Following the recent shift to a 4-column layout in the theme picker [1],
this commit focuses on updating a little bit the size and quality of
theme screenshot images. Providing a clearer and more visually appealing
presentation of the themes.
task-3648576
closesodoo/design-themes#758
Related: odoo/odoo#149989
Signed-off-by: Soukéina Bojabza (sobo) <sobo@odoo.com>