Under Settings > Document templates, you’ll find 5 different layouts to choose from for the documents you create using LSP.expert. For each of these templates, you can choose a colour and download an example (using the Preview button).
To know how to customize your documents even more, see this article: https://help.lsp.expert/en/articles/1863932-can-i-customize-the-documents-i-create-with-lsp-expert.
If you're using LSP.expert V1
In Settings > Document templates, you’ll find 5 different layouts to choose from for your invoices and quotes. You can download an example for each of them and choose your color.