How can I convert a coloured icon to a monochrome one?

Ok, I invested some time and did it myself. What I did was basically use GIMP's Alpha to Selection tool to select the non-transparent parts of the image. Then I chose the Bucket Fill tool to apply a homogenous grey colour to the icon. All other refinements I did manually with the Paintbrush tool.

These are the raw icons:

(programicon_old.png) (programicon_scan.png)

This is what the indicator now looks like:

In order to install the icons you will have to compile GoldenDict from source:

• Clone the GitHub repository

  git clone https://github.com/goldendict/goldendict.git
  

• Navigate to ./goldendict/icons and replace programicon_old.png and programicon_scan.png with the updated icons

• Follow the instructions in README.md to compile and install GoldenDict