Yes, it's possible to convert these themes into "semi-transparent" themes but it would require some work (probably 2-3 hours). Especially some shortcodes, icons and shadow images are tricky to convert. You can start with following css code:
.header_color,.header_color .site-background,.header_color .first-quote,.header_color .related_image_wrap,.header_color .gravatar img,.header_color .comment-reply-link,.header_color .inner_slide .numeric_controls a,.header_color .hr_content,.header_color .news-thumb,.header_color .post-format-icon,.header_color .ajax_controlls a
{
background-color:transparent;
}
.socket_color div .avia_table tr.description_row td,.socket_color div .avia_table tr.pricing-row td,.socket_color tr.pricing-row .avia-table-icon,.socket_color tr.description_row .avia-table-icon,.socket_color .css_3_hover
{
background:tranparent;
}
#wrap_all{
background: rgba(0,0,0,0.6);
}
- it will remove the bg color from most "content" elements and add a semi transparent (black) background to the surrounding container.