how can I change the default color displayed in Big Grid 1?

Hi,
The background color is grabbed from the theme accent color you set in the colors section of the theme panel. For a custom color you can use css as @Aronfly described.
Alternatively you can add the code in the theme panel custom css box to not have to edit style.css on every update.
Thank you!

Hi,

Style.css is in the main theme folder, but any css modification is easier to make in the theme panel-> custom css box.
Try to add this code in the custom css box in the theme panel:
.td-grid-style-4 .entry-title{
background-color: rgba(0, 102, 153, 0.7)!important;
}

Thank you!

Hi,

That is not an error but a simple warning. every time you use !important in your css code this warning will show up, but it can be ignored as it warns us not to use !important unless it;s absolutely necessary. you can remove the !important tag if you want and check if the code works without it. We usually add it as css needs to re-write the previous code and sometimes it cannot without !important so we add it in. Try it like so and if it works the error will be gone. If it does not, then simply ignore the message:
.td-grid-style-4 .entry-title{
background-color: rgba(0, 102, 153, 0.7);
}

Thank you!