can I apply different emphasis to certain level of numbering in writer, for example bold the first level but remain the sub level as normal?
This question needs more detail. Numbered list or chapter/outline numbering? What are you trying to do?
“Numbering” may refer to chapter structure or numbered list. Since everything in LO Writer is a matter of style, your question targets the involved styles for both topics.
Chapter structure
A paragraph becomes a member of chapter numbering when it received Heading 1 to Heading 10 paragraph style. Level 1 is chapter, 2 sub-chapter, and so on.
To give a different emphasis to level, edit the Heading x (x from 1 to 10; only consider those you effictively use)…
Heading x is for levels inside the document. If you want to differentiate levels in the table of contents, edit the Contents x style which parallel the Heading x styles.
Numbered or bulleted lists
This is a bit more tricky because the same paragraph style is applied to all levels. The emphasis may only differ on rthe numbering or bullet.
Lists are a difficult concept in Writer. Read this answer to have an idea of the complexity.
However, if you’re only using the [simplified] feature through the toolbar buttons, default numbering emphasis can be customised through Format → Bullets & Numbering. Go to the Customise tab. You’ll find a Character Style drop-down menu which applies a character style to the selected level.
This means you must first define the character styles you need because the standard list contains styles intended for other topics.
Summary
Get familiar with styles and understand the differences between paragraph, character and list styles. The latter one is a misnomer because it controls only the bullet/number of the list.
Read the chapters about styles in the manuals. Styles are the keys to control Writer.
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for long answer please refer the answer above (for you to understand what you doing with short answer) , for short answer: 1. press F11 to bring up the styles and formatting panel, go to second tab (character styles), right click to add new style, edit the style according to your need. 2. go to format > bullets and numberings…>customize > character style , select the character style you just create.