Set the Style in the Subtitle File Itself.

For SRT files, the options for the subtitle styles are limited.

Basic styling can be set using HTML markups like <b>, <i>, <u>, and <font>.

subtitle.srt

1
00:00:0,000 --> 00:00:2,000
This is the <b><i>first</i></b> sentence
 
2
00:00:2,000 --> 00:00:4,000 
This is the second sentence
 
3
00:00:4,000 --> 00:00:7,000
This is the third sentence
 
4
00:00:7,000 --> 00:00:10,000
This is the forth sentence