i think i cant manage like this .
take a look at this for example:

for this: <h4 class="no-margin-top">
in template i have: adsmanager-grid h4 {
margin-bottom: 5px;
padding: 5px;
background-color: #000000;

and the fot is getting from template....


and in CB i have:

for the same h4class:
cb_template h1, .cb_template h2, .cb_template h3, .cb_template h4, .cb_template h5, .cb_template h6, .cb_template .h1, .cb_template .h2, .cb_template .h3, .cb_template .h4, .cb_template .h5, .cb_template .h6 {
font-family: inherit;
font-weight: 500;
line-height: 1.1;
color: inherit;
}

so the tabadsmanager is running directly in CB.. wil take cb css

i wil change the font in cb
dont knwo with other..because are extra css codes that modify ..like padding.. etc
or that border box..

Some works...some no.. because adsmanager tab runs in Community builder ..and extra css overiding my template.
And if change to have good in adsmanager..i ll have ome probs with cb.

Second issue:
so if a user choose first ..because the user choose first the duration of 15 days...
i think is not ok to be displayed the option for top with a bigger number of days

You are right... using !important will broke cb ...
but .. CB changes even the font style..etc... this means that i must set ...all in adsmanager css like in joomla template to search all css that overrides
/* font-family: inherit; */
/* font-weight: 500; */
/* line-height: 1.1; */
/* color: inherit; */
etc

and for example this:
cb_template * {
box-sizing: border-box;

is changing the width ... and if i disable in cb ..i will have problems

i have to add: box-sizing:none with important ?