Questioner Info
Description

First of all, thank you for this awesome theme.
If possible, kindly help me change the button colour. The same colour is used in highlighting buttons inside Inkscape. I would happy if it is closer to white. Thanks.
Hi,
Please try the latest Materia by manually installing it. As written in the README, Ubuntu packages are very outdated. In the latest Materia, the activated button has a much lighter background-color :)

Please tell me if it's still too dark for you.
Many thanks for the response. I was customizing Materia a bit with #03a9f4 colour. I would like to change grey to a lighter shade of #03a9f4. I would be happy to know where to find it in gtk.css.
I would like to install the new Materia. Just not sure how to remove the old one. Thanks.
I would be happy to know where to find it in gtk.css.
gtk.css is for GTK3, and what you are showing in OP is GTK2. GIMP and Inkscape are using GTK2. So you need to edit the GTK2 theme.
I would like to install the new Materia. Just not sure how to remove the old one.
Usually you don't need to remove it. You can overwrite it while installing.
Try this:
Clone the source and move to materia-theme dir in your terminal:
git clone --depth 1 https://github.com/nana-4/materia-theme
cd materia-theme
Open src/gtk-2.0/assets.svg in your text editor and edit this line to:
<use xlink:href="#base-button-box" fill="#03a9f4" fill-opacity="0.2"/>
(you may want to adjust fill-opacity as well.)
Run ./render-assets.sh gtk2 in the terminal.
sudo ./install.sh in the terminal.I have cloned materia-theme into my PC. I am in process of editing the gtk2 assets. Can the same be done for gtk asset as well ? Thanks.
We don't use SVG/PNG assets for activated buttons in GTK3.
I see. I am really new to this. Thanks.
It's fine :) So can we close this now?
Sure, please. :)