A drawable can either be an xml file, a vector graphic imported into Android Studio or a bitmap image. To learn more about drawables on Android, please read the documantation about drawables.
In your themes.xml a drawable resource is defined in the same way as a reference to a drawable in the rest of Android Studio. It always starts with @drawable/ + DRAWABLE_NAME. Where DRAWABLE_NAME is the name of your drawable.
A color can either be a value defined in the res→values→colors.xml or be directly written in the themes.xml file with # + COLOR. To use a color from res→values→colors.xml, you have to write @color/ + COLOR_NAME. To see how you can define a COLOR value directly in the themes.xml, read the documentation about colors.
A boolean value can either be a value defined in res→values→booleans.xml or be directly written in the themes.xml file. To write the value directly into the themes.xml file, simply write true or false. To use a value res→values→booleans.xml, write @bool/ + BOOLEAN_NAME. For more information about booleans read the documentation.
A dimension depicts a size or distance in the UI. This size can be defined with different units like pixel, DPI, Inches or others. You can either use a dimension value defined in res→values→dimens.xml by writing @dimen/ + DIMENSION_NAME or directly write the value with a unit into the themes.xml file. You can find the supported units, their explanation and mor info in the documentation about dimension resources.
Strings can either be written directly in the themes.xml or taken from the res→values→strings.xml file. To use a value from res→values→strings.xml, write @string/ + *STRING_NAME. You can find more information about strings in the documuntation.