Dropping the .cvs files renders a new splashscreen. I am at a loss here, this is exactly what i have been looking for (except for the dark background) (: Any idea where to start poking? 

The console goes: 

/private/var/folders/y7/bwn1ddp96ts4b3j11k_5_gvw0000gn/T/AppTranslocation/60A6EA7B-62E9-4EBA-B5D5-C570023EC015/d/Left.app/Contents/Resources/app/sources/scripts/lib/theme.js:121 Theme Incomplete SVG Theme TypeError: Cannot read property 'getAttribute' of null
    at extract (/private/var/folders/y7/bwn1ddp96ts4b3j11k_5_gvw0000gn/T/AppTranslocation/60A6EA7B-62E9-4EBA-B5D5-C570023EC015/d/Left.app/Contents/Resources/app/sources/scripts/lib/theme.js:110)
    at parse (/private/var/folders/y7/bwn1ddp96ts4b3j11k_5_gvw0000gn/T/AppTranslocation/60A6EA7B-62E9-4EBA-B5D5-C570023EC015/d/Left.app/Contents/Resources/app/sources/scripts/lib/theme.js:51)
    at Theme.load (/private/var/folders/y7/bwn1ddp96ts4b3j11k_5_gvw0000gn/T/AppTranslocation/60A6EA7B-62E9-4EBA-B5D5-C570023EC015/d/Left.app/Contents/Resources/app/sources/scripts/lib/theme.js:31)
    at FileReader.reader.onload (/private/var/folders/y7/bwn1ddp96ts4b3j11k_5_gvw0000gn/T/AppTranslocation/60A6EA7B-62E9-4EBA-B5D5-C570023EC015/d/Left.app/Contents/Resources/app/sources/scripts/lib/theme.js:71)
extract @ /private/var/folders/y7/bwn1ddp96ts4b3j11k_5_gvw0000gn/T/AppTranslocation/60A6EA7B-62E9-4EBA-B5D5-C570023EC015/d/Left.app/Contents/Resources/app/sources/scripts/lib/theme.js:121
/private/var/folders/y7/bwn1ddp96ts4b3j11k_5_gvw0000gn/T/AppTranslocation/60A6EA7B-62E9-4EBA-B5D5-C570023EC015/d/Left.app/Contents/Resources/app/sources/scripts/lib/theme.js:32 Theme Not a theme undefined
Theme.load @ /private/var/folders/y7/bwn1ddp96ts4b3j11k_5_gvw0000gn/T/AppTranslocation/60A6EA7B-62E9-4EBA-B5D5-C570023EC015/d/Left.app/Contents/Resources/app/sources/scripts/lib/theme.js:32

You shouldn't drop any cvs file on Left, the only theme format is svg. If you drop a svg file on the left window, it should apply the theme.

Sorry if i was not clear and to bother you again – (english is far from my first language).

That is the way i am trying to do it – dragging and dropping the .cvs file into the application window. Nor does the ⇧⌘O Open theme seem to work for me.  Could it be an OS X issue? I am running OS X 10.14.6 Mojave.

SOLVED IT!