Just want some info on where I can change the theme of Writer - it is dark grey background under all the menu elements, as well as how can I set a default font for typing when I start a new doc? It is set to Liberation Serif and I want to change it.
You can change the colour for the background of the top in Tools > Options > LibreOffice > Personalisation
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I don’t think you can change the menu background colour unless you do it in the operating system. There might be work in hand but I don’t know the extent, see Trying to change the colors/theme of the app - #2 by mikekaganski
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For Default Font
Click Tools > Options > LibreOffice Writer > Basic fonts (Western) and set your default fonts there. If you use a complex language, e.g. Hindi, Arabic then set the fonts in the relevant Basic fonts pane
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If you need any other customisations, then you need to create a default template, Creating and Changing Default and Custom Templates