Stela works best with black & white logo as home page backgorund of header is not white as well as background on other pages (green).

There are 2 options:

1) disable converting to white logo
- open file oc-content/themes/stela/css/style.css
- find line starting with:
#header-bar .logo img {
- in this line remove code:
filter: brightness(0) invert(1);
- now, logo will look like original uploaded

2) You want to have white logo, but it seems whole logo square is white
- prepare logo with transparent background
- go to oc-admin > settings > media
- uncheck "Force JPEG extension"
- go to theme settings (header logo) and upload your logo
- logo will be saved as jpeg, however transparent background persist
- once logo is uploaded, you may check "Force JPEG extension" again


Recommended image dimension
Stela osclass theme expect bit larger images as in default osclass settings. Only change is related to thumbnail images.
Go to oc-admin > settings > media

Recommended image dimension:
Thumbnail: 320x240
Preview: 480x360
Normal: 800x600

Aspect ratio is 4:3 so you may set also larger size for images when this ratio is kept.

Save changes and regenerate thumbnails.